In some cases one may want to use GCC to build something when its
runtimes or GCC at runtime are not required.

Introduce an optional argument for USE_GCC that indicates GCC is
only required at build time.

Examples for the new syntax are USE_GCC=yes:build, USE_GCC=9:build,
or USE_GCC=11+:build.

Submitted by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
PR: 211154
Differential Revision: https://reviews.freebsd.org/D7223