utils/macros.h: Refactor the GCC-ish compiler support.

Fully define `GCC_VERSION_P' early, and then use it without any other
guard to define the GCC-specific hacks.  This way, it's easy to support
other compilers which behave similarly, by defining an appropriate
`FOO_VERSION_P' macro, and then adding appropriate extra alternatives to
the individual guards.

No functional changes.