Merge branch 'ab/c99-variadic-macros'

Remove the escape hatch we added when we introduced the weather
balloon to use variadic macros unconditionally, to make it official
that we now have a hard dependency on the feature.

* ab/c99-variadic-macros:
  C99: remove hardcoded-out !HAVE_VARIADIC_MACROS code
  git-compat-util.h: clarify GCC v.s. C99-specific in comment