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