Recipe to address cyclic deps for bulkbuild?

[Mayuresh <mayuresh%acm.org@localhost>]

After setting `ccache' as PKGSRC_COMPILER for pbulk based builk builds,
came across a cyclic dependency between gmake and ccache.

Solved it by setting PKGSRC_COMPILER=gcc in gmake's Makefile.

But that sounds a bit ad-hoc to me. Looking for a way to at least do it in
mk.conf instead of the Makefile.

Just like we can set PKG_OPTIONS.pkgbase, can package specific values be
provided for other variables - say PKGSRC_COMPILER.

Such ability will also be useful to, e.g., tweak MAKE_JOBS for certain
packages (say rust, though it's handled in the the package now, IMO it's
more flexible to leave it to individual setups).

-- 
Mayuresh