git-compat-util.h: support variadic macros with the XL C compiler
git-compat-util.h: support variadic macros with the XL C compiler

When the XL C compiler is run with an appropriate language level or
suboption, it defines a feature test macro to indicate support for
variadic macros by defining __C99_MACRO_WITH_VA_ARGS C preprocessor
macro.

This was tested on z/OS, but it should also work on AIX according
to IBM documentation.

Signed-off-by: David Michael <7bb1caa8d2fd92e201656506e5ceb6def55b478e@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git