Created attachment 674383 [details]
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -m64 -m64 -m64 -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -I. -I./Include -I/Users/sam/Gentoo/tmp/usr/include -I/Users/sam/Gentoo/tmp/usr/include -DPy_BUILD_CORE_BUILTIN -c ./Modules/_abc.c -o Modules/_abc.o
./Modules/posixmodule.c:8436:15: error: implicit declaration of function 'sendfile' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
ret = sendfile(in, out, offset, &sbytes, &sf, flags);
Needed to set ac_cv_enable_implicit_function_declaration_error=no and -Wno-implicit-function-declaration.
Note: you can't set -Wno-implicit-function-declaration globally for the whole bootstrap, otherwise some configure tests will end up failing when they check if functions are declared.
If this was the stage1 failure, I've tackled that by simply building 3.7.8. Version in tree is also 3.7.8, so that should work too.