Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add WITH_GCC alongside WITHOUT_GCC.
Remove the comment from WITHOUT_GCC about this not working without an
alternate toolchain, we have clang now.

Implement WITH{,OUT}_{GCC,BINUTILS} to provide finer-grained control
over building gcc and binutils. They default to true, unless
MK_TOOLCHAIN is no.

Reviewed by: ru@