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.
