USE="clang" sudo emerge btsocket


 creating build/temp.linux-x86_64-2.7/src
btsocket-0.0.1-r11: x86_64-pc-linux-gnu-clang -O2 -pipe -fPIC -I/usr/include/python2.7 -c src/btsocket.c -o build/temp.linux-x86_64-2.7/src/btsocket.o
btsocket-0.0.1-r11: unable to execute 'x86_64-pc-linux-gnu-clang': No such file or directory
btsocket-0.0.1-r11: error: command 'x86_64-pc-linux-gnu-clang' failed with exit status 1

This is because the function clang-setup-env will set the CC to
"${CHOST}-clang". For host, there is no such wrapper, so the error happens.