with glibc 2.23, I run
emerge-falco update_engine

I got
112/115] CXX aosp/system/update_engine/libupdate_engine.p2p_manager.o
FAILED: x86_64-cros-linux-gnu-g++ -MMD -MF aosp/system/update_engine/libupdate_engine.p2p_manager.o.d -D__CHROMEOS__ -D_FILE_OFFSET_BITS=64 -D_POSIX_C_SOURCE=199309L -DUSE_BINDER=0 -DUSE_DBUS=1 -DUSE_HWID_OVERRIDE=0 -DUSE_LIBCROS=1 -DUSE_MTD=0 -DUSE_POWER_MANAGEMENT=1 -DUSE_WEAVE=0 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -I/build/falco/tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/aosp/system -I/build/falco/tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/aosp/system/update_engine/client_library/include -Iaosp/system/update_engine/libupdate_engine.gen/include -Igen/include -I/build/falco/tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/platform2 -I/build/falco/tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/platform -I/build/falco/usr/include -Igen -g -ffunction-sections -Wall -Wextra -Werror -Wno-unused-parameter -Wall -Wno-psabi -ggdb3 -fstack-protector-strong -Wformat=2 -fvisibility=internal -Wa,--noexecstack -Werror --sysroot=/build/falco -DUSE_RTTI_FOR_TYPE_TAGS -Wno-c++11-extensions -Wno-unused-local-typedefs -DBASE_VER=381699 -pthread -I/build/falco/usr/include/chromeos -I/build/falco/usr/include/debugd-client -I/build/falco/usr/include/session_manager-client -I/build/falco/usr/include/base-381699 -I/build/falco/usr/include/glib-2.0 -I/build/falco/usr/lib64/glib-2.0/include -I/build/falco/usr/include/dbus-1.0 -I/build/falco/usr/lib64/dbus-1.0/include -I/build/falco/usr/include/nss -I/build/falco/usr/include/nspr -I/build/falco/usr/include/power_manager-client -I/build/falco/usr/include/update_engine-client -I/build/falco/usr/include/shill-client -I/build/falco/usr/include/xz-embedded -fPIE -fno-strict-aliasing -Wnon-virtual-dtor -std=gnu++11 -DVCSID='"0.0.3-r1912-ffbb9359abab41584a003c386bbc9bb146b8c1ca"' -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -O2 -pipe -O2 -pipe -clang-syntax -O2 -pipe -march=corei7 -g -fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables -clang-syntax -clang-syntax -fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables  -c ../../../../../../../tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/aosp/system/update_engine/p2p_manager.cc -o aosp/system/update_engine/libupdate_engine.p2p_manager.o
In file included from ../../../../../../../tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/aosp/system/update_engine/p2p_manager.cc:23:
In file included from /build/falco/tmp/portage/chromeos-base/update_engine-0.0.3-r1912/work/update_engine-0.0.3/aosp/system/update_engine/p2p_manager.h:20:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/include/g++-v4/string:38:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/include/g++-v4/x86_64-pc-linux-gnu/bits/c++config.h:430:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/include/g++-v4/x86_64-pc-linux-gnu/bits/os_defines.h:39:
/build/falco/usr/include/features.h:148:3: error: "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Werror,-W#warnings]
# warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
  ^
1 error generated.