Package Makefiles may now directly include compiler.mk.
* Don't include compiler.mk within bsd.prefs.mk any longer. It was only
included for the purposes of defining CC_VERSION. Packages that want
to test the value of CC_VERSION should now first include
"../../mk/compiler.mk". Any GCC_REQD statements in package Makefiles
should be set before compiler.mk is included.
* Simpllfy pkgsrc/mk/compiler/*.mk files as a result of not needing to
be included indirectly by bsd.prefs.mk. We remove the special handling
associated with detecting whether the file was included from within
bsd.prefs.mk. These files are now much more straightforward to write
and understand.
* G/C the BSD_PREFS_MK stack mechanism as the only users (compiler/*)
no longer need it.
* Ensure that directories are prepended to the PATH only from within
bsd.pkg.mk.
