gcc50: Switch to SVN over weekly snapshots for latest

gcc50: Switch to SVN over weekly snapshots for latest

A dports bulk run revealed at least 3 major issues, all of which have
already been fixed upstream.  Two were found in 8 February snapshot, but
the third still hasn't been release.  Since the snapshots are just
the packaged Subversion repo without the .svn directory, I'm switching
over to SVN directly until the prerelease.

With here, disable extra diagnostic checks which slow the compiler down
considerably.