git-compat-util.h: GCC deprecated message arg only in GCC 4.5+

https://gcc.gnu.org/gcc-4.5/changes.html says

  The deprecated attribute now takes an optional string argument, for
  example, __attribute__((deprecated("text string"))), that will be
  printed together with the deprecation warning.

While GCC 4.5 is already 12 years old, git checks for even older
versions in places. Let's not needlessly break older compilers when
a small and simple fix is readily available.

Signed-off-by: Alejandro R. Sedeño <asedeno@mit.edu>
Signed-off-by: Alejandro R Sedeño <asedeno@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -189,9 +189,12 @@ struct strbuf;
 #define _NETBSD_SOURCE 1
 #define _SGI_SOURCE 1
 
-#if defined(__GNUC__)
+#if GIT_GNUC_PREREQ(4, 5)
 #define UNUSED __attribute__((unused)) \
 	__attribute__((deprecated ("parameter declared as UNUSED")))
+#elif defined(__GNUC__)
+#define UNUSED __attribute__((unused)) \
+	__attribute__((deprecated))
 #else
 #define UNUSED
 #endif