USE_PACKAGES and gmake

[Ted Unangst]

I was building a custom flavor of a port, using USE_PACKAGES, and some
dependency required gmake to build.  The package was used instead, but
it looked like setting USE_GMAKE=Yes would have installed gmake.  Is
that correct?  I already have gmake installed, I'm curious what would
have happened, and more importantly, what should happen in this case.