dfa: port to older GCC Problem reported by Assaf Gordon in: http://lists.gnu.org/archive/html/bug-gnulib/2017-01/msg00103.html * modules/c99: New module. This merely attempts to use the latest C version, which should be enough to solve this particular problem. The idea is to document which Gnulib modules assume C99 or later. * modules/dfa (Depends-on): Add it.