Add a new option, GCC, that controls the use of GCC instead of the
system compiler (usually clang these days) as we have been doing since
r334935 in 2013-11-26. And make that the default, both since there've
been user reports about Wine not working properly with the toolchain
in FreeBSD 12 and there may be other challenges given that upstream
and in other distributions GCC is being used all along. [1]

Also backport part of r488645 | gerald | 2018-12-29 01:53:56 from
emulators/wine-devel:

Create a proper USES block in Makefile, moving the existing USES
statement there.

PR: 231010 [1]