Heads-up: Removing gperf from base

gperf is a tool that generates perfect hash functions. It is used as a
bootstrap tool when building gcc, and by a few ports.

Today we unconditionally build and install gperf, including on clang
architectures where it's not required. I plan to switch this so that
gperf is only built when gcc is (that is, make it conditional on
MK_GCC).

The ports tree will use the devel/gperf port if gperf is not available
from the base system. A ports exp-run is planned to verify this works,
and I expect to commit the change after that's confirmed.

PR: https://bugs.freebsd.org/194103
Review: https://reviews.freebsd.org/D886

-Ed