On FreeBSD, the gcc manpages until Release 5.1 mentioned using
the -D_THREAD_SAFE define when using -pthread. This has been obsoleted
and the gcc manpages from Release 5.2 onwards have this removed.

Now we finally remove this historic relic here, too.

Change-Id: I00a5b688c56f46b938c0806fb44b72d5afe5079e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>