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 <fedora.dm0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -750,7 +750,7 @@ void git_qsort(void *base, size_t nmemb, size_t size,
 #endif
 #endif
 
-#if defined(__GNUC__) || (_MSC_VER >= 1400)
+#if defined(__GNUC__) || (_MSC_VER >= 1400) || defined(__C99_MACRO_WITH_VA_ARGS)
 #define HAVE_VARIADIC_MACROS 1
 #endif