It needs a GNU _compatible_ C compiler. You can still use LLVM's Clang.
I'm guessing that it invokes /usr/include which on Linux is probably not neutral to CPP or gnu-isms.
Your point about "compatible" is well made. I should have been clearer.
Wasn't gentoo born as Linux userspace on BSD kernel at one point?
Gentoo was born as linux. Portage, the package manager, was inspired from BSD ports. Maybe they tried to experiment with also using a BSD kernel later, as did Debian, but it was only a side project.
I think Gentoo was always about Linux with BSD-style ports. They had some additional "flavors" that switched Linux to kernels from FreeBSD, NetBSD or OpenBSD, but it seems those alternative Gentoo distros are deprecated for at least a few years already.