Try to enable C99 in configure, but do not rely on it (yet).
Based on recent discussion it seems possible that we might start to
rely on more of C99. A prerequisite for that is enabling support for
that on used compilers.
Let's see on which buildfarm members autoconf's AC_PROG_CC_C99() is
sufficient to do so. There's probably at least one member where the
compiler is too old, but that'd probably be OK.
If we go for this permanently we'd likely want to clean out / up a few
other configure tests.
Note this does not touch the msvc build infrastructure, which'd need
separate treatment.
Discussion: https://postgr.es/m/
20180815222401.kxsupl5zie2jgi4x@alap3.anarazel.de