Merge branch 'jk/weather-balloon-require-variadic-macro'

Merge branch 'jk/weather-balloon-require-variadic-macro'

We've carried compatibility codepaths for compilers without
variadic macros for quite some time, but the world may be ready for
them to be removed.  Force compilation failure on exotic platforms
where variadic macros are not available to find out who screams in
such a way that we can easily revert if it turns out that the world
is not yet ready.

* jk/weather-balloon-require-variadic-macro:
  git-compat-util: always enable variadic macros