databases/mysql57-client: Fix the build with GCC-based architectures
This patch adds a condition to FreeBSD part of MySQL CMake-scripts
to recognize gcc as well as Clang, while taking care of Unsupported compilers too.

This is an enhancement on the initial patch by Piotr Kubaj <pkubaj@anongoth.pl> that
committed by linimon on r488087.

PR: 234172
Sponsored by: The FreeBSD Foundation