It often fails to build the first time and succeeds on the 2nd attempt. For example:
https://luci-milo.appspot.com/buildbot/chromeos/cave-paladin/1726

https://luci-milo.appspot.com/buildbot/chromeos/chell-paladin/2687


"
imageloader-0.0.1-r859: >>> Configuring source in /build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader ...
imageloader-0.0.1-r859: /build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2/common-mk/platform2.py --libdir=/usr/lib64 --use_flags=abi_x86_64 amd64 cros-debug elibc_glibc kernel_linux multilib userland_GNU --jobs=32 --action=configure --cache_dir=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/build /build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp --disable_incremental
imageloader-0.0.1-r859: Package devmapper was not found in the pkg-config search path.
imageloader-0.0.1-r859: Perhaps you should add the directory containing `devmapper.pc'
imageloader-0.0.1-r859: to the PKG_CONFIG_PATH environment variable
imageloader-0.0.1-r859: No package 'devmapper' found
imageloader-0.0.1-r859: gyp: Call to '/build/chell/build/bin/pkg-config devmapper libbrillo-395517 libchrome-395517 libcrypto libminijail protobuf-lite --cflags' returned exit status 1.
imageloader-0.0.1-r859: Traceback (most recent call last):
imageloader-0.0.1-r859:   File "/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2/common-mk/platform2.py", line 333, in <module>
imageloader-0.0.1-r859:     main(sys.argv[1:])
imageloader-0.0.1-r859:   File "/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2/common-mk/platform2.py", line 329, in main
imageloader-0.0.1-r859:     getattr(p2, options.action)(options.args)
imageloader-0.0.1-r859:   File "/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2/common-mk/platform2.py", line 239, in configure
imageloader-0.0.1-r859:     % ' '.join(map(repr, gyp_args)))
imageloader-0.0.1-r859: AssertionError: Error running: 'gyp' '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp' '--format=ninja' '--include=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2/common-mk/common.gypi' '--depth=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2' '--toplevel-dir=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2' '--generator-output=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/build' '-DOS=linux' '-Dpkg-config=/build/chell/build/bin/pkg-config' '-Dsysroot=/build/chell' '-Dlibdir=/usr/lib64' '-Dbuild_root=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/build' '-Dplatform2_root=/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform2' '-Dlibbase_ver=395517' '-Dclang_syntax=' '-Denable_exceptions=0' '-Dexternal_cflags=-O2 -pipe -O2 -pipe -march=corei7 -g -fno-exceptions -fno-unwind-tables   -fno-asynchronous-unwind-tables  -clang-syntax -clang-syntax' '-Dexternal_cxxflags=-O2 -pipe -O2 -pipe -O2 -pipe -march=corei7 -g -fno-exceptions -fno-unwind-tables   -fno-asynchronous-unwind-tables  -clang-syntax -clang-syntax' '-Dexternal_cppflags= -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE' '-Dexternal_ldflags=-Wl,-O1 -Wl,-O2 -Wl,--as-needed' '-DUSE_abi_x86_64=1' '-DUSE_amd64=1' '-DUSE_multilib=1' '-DUSE_cros_debug=1' '-DUSE_kernel_linux=1' '-DUSE_elibc_glibc=1' '-DUSE_userland_GNU=1'
imageloader-0.0.1-r859:  * ERROR: chromeos-base/imageloader-0.0.1-r859::chromiumos failed (configure phase):
imageloader-0.0.1-r859:  *   (no error message)
imageloader-0.0.1-r859:  * 
imageloader-0.0.1-r859:  * Call stack:
imageloader-0.0.1-r859:  *     ebuild.sh, line   93:  Called src_configure
imageloader-0.0.1-r859:  *   environment, line 3969:  Called platform_src_configure
imageloader-0.0.1-r859:  *   environment, line 3572:  Called platform_configure '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp'
imageloader-0.0.1-r859:  *   environment, line 3530:  Called platform 'configure' '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp'
imageloader-0.0.1-r859:  *   environment, line 3525:  Called die
imageloader-0.0.1-r859:  * The specific snippet of code:
imageloader-0.0.1-r859:  *       "${cmd[@]}" || die
imageloader-0.0.1-r859:  * 
imageloader-0.0.1-r859:  * If you need support, post the output of `emerge --info '=chromeos-base/imageloader-0.0.1-r859::chromiumos'`,
imageloader-0.0.1-r859:  * the complete build log and the output of `emerge -pqv '=chromeos-base/imageloader-0.0.1-r859::chromiumos'`.
imageloader-0.0.1-r859:  * The complete build log is located at '/build/chell/tmp/portage/logs/chromeos-base:imageloader-0.0.1-r859:20170929-191801.log'.
imageloader-0.0.1-r859:  * For convenience, a symlink to the build log is located at '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/temp/build.log'.
imageloader-0.0.1-r859:  * The ebuild environment file is located at '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/temp/environment'.
imageloader-0.0.1-r859:  * Working directory: '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader'
imageloader-0.0.1-r859:  * S: '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader'
imageloader-0.0.1-r859: >>> Failed to emerge chromeos-base/imageloader-0.0.1-r859 for /build/chell/, Log file:
imageloader-0.0.1-r859: >>>  '/build/chell/tmp/portage/logs/chromeos-base:imageloader-0.0.1-r859:20170929-191801.log'
imageloader-0.0.1-r859: 
imageloader-0.0.1-r859:  * Messages for package chromeos-base/imageloader-0.0.1-r859 merged to /build/chell/:
imageloader-0.0.1-r859: 
imageloader-0.0.1-r859:  * ERROR: chromeos-base/imageloader-0.0.1-r859::chromiumos failed (configure phase):
imageloader-0.0.1-r859:  *   (no error message)
imageloader-0.0.1-r859:  * 
imageloader-0.0.1-r859:  * Call stack:
imageloader-0.0.1-r859:  *     ebuild.sh, line   93:  Called src_configure
imageloader-0.0.1-r859:  *   environment, line 3969:  Called platform_src_configure
imageloader-0.0.1-r859:  *   environment, line 3572:  Called platform_configure '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp'
imageloader-0.0.1-r859:  *   environment, line 3530:  Called platform 'configure' '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader/imageloader.gyp'
imageloader-0.0.1-r859:  *   environment, line 3525:  Called die
imageloader-0.0.1-r859:  * The specific snippet of code:
imageloader-0.0.1-r859:  *       "${cmd[@]}" || die
imageloader-0.0.1-r859:  * 
imageloader-0.0.1-r859:  * If you need support, post the output of `emerge --info '=chromeos-base/imageloader-0.0.1-r859::chromiumos'`,
imageloader-0.0.1-r859:  * the complete build log and the output of `emerge -pqv '=chromeos-base/imageloader-0.0.1-r859::chromiumos'`.
imageloader-0.0.1-r859:  * The complete build log is located at '/build/chell/tmp/portage/logs/chromeos-base:imageloader-0.0.1-r859:20170929-191801.log'.
imageloader-0.0.1-r859:  * For convenience, a symlink to the build log is located at '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/temp/build.log'.
imageloader-0.0.1-r859:  * The ebuild environment file is located at '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/temp/environment'.
imageloader-0.0.1-r859:  * Working directory: '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader'
imageloader-0.0.1-r859:  * S: '/build/chell/tmp/portage/chromeos-base/imageloader-0.0.1-r859/work/imageloader-0.0.1/platform/imageloader'
"