Build gperf only if we're using g++ (not clang++)
gperf is used as a build tool for g++ and is not needed for Clang
architectures. Ports and third-party software that need it can use the
up-to-date devel/gperf port.
PR: 194103 (exp-run)
Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D886