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