Enable building tbb with non-Apple compilers.

Without this change, when installing with --cc=gcc-4.8, you'd get the error:

==> make tbb_build_prefix=BUILDPREFIX compiler=gcc-4.8 arch=intel64
Created ./build/BUILDPREFIX_release and ..._debug directories
/Applications/Xcode.app/Contents/Developer/usr/bin/make -C "./build/BUILDPREFIX_debug"  -r -f ../../build/Makefile.tbb cfg=debug
../../build/common.inc:103: *** "gcc-4.8" is not supported on macos. Add build/macos.gcc-4.8.inc file with compiler-specific settings. .  Stop.
make: *** [tbb] Error 2

If I tried instead to use --cc=gcc, it'd just use clang anyways.