git-compat-util.h: #undef (v)snprintf before #define them

When we detect that vsnprintf / snprintf are broken, we #define them
to an alternative implementation.  On OS X, stdio.h already
re-define them in `git-compat-util.h'.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tsuna

git-compat-util.h

@@ -374,9 +374,15 @@ extern FILE *git_fopen(const char*, const char*);
 #endif
 
 #ifdef SNPRINTF_RETURNS_BOGUS
+#ifdef snprintf
+#undef snprintf
+#endif
 #define snprintf git_snprintf
 extern int git_snprintf(char *str, size_t maxsize,
 			const char *format, ...);
+#ifdef vsnprintf
+#undef vsnprintf
+#endif
 #define vsnprintf git_vsnprintf
 extern int git_vsnprintf(char *str, size_t maxsize,
 			 const char *format, va_list ap);