Andy Wingo <wingo@pobox.com> writes:

> Is there anyone who compiles Guile with a compiler that does not support
> C99?  If so, please give platform and compiler.

Not really the question you asked, and pretty fuzzy, but an anecdote
about a real situation I am aware of:

  In 2015--2016, there was some C code that was supposed to be very
  portable and a notion that using C99 features would not be a problem
  in any environment (as in "In 2015, it's beyond lame not to support
  C99".)  It turned out that it was almost entirely no problem, except
  for one build environment that was using an old compiler, I think from
  MS.  That compiler could handle almost all of C99, except mixed
  declarations and code.

Based on that, I do not expect problems with guile requiring C99,
especially if you refrain from mixed decls/code.