Enables only using GCC

In some cases you do not want to use the built-in optimizing linker. You
can only use GCC by (for example):

    EnvironmentLinker:	/usr/bin/gcc::-g -lmysqlclient|... (other
    linkers)

Note that :: ensures the options are passed to gcc.

Co-authored-by: Mart Lubbers <mart@martlubbers.net>
1 job for support-only-using-gcc in 53 seconds
Status Job ID Name Coverage
  Test
passed #39345
test

00:00:53