WITH_/WITHOUT_CXX: update description

Contrary to the previous description WITHOUT_CXX does not disable
/usr/bin/c++, which is just a link to Clang. We also no longer have
gperf.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 145a574fa1a9fa6732a7917778ed3185e990d1dd)