libxl: fix build for gcc prior to 4.3

So far all we (explicitly) require is gcc 3.4 or better, so we
shouldn't be unconditionally using features supported only by much
newer versions.

Short of a proper replacement, use the "deprecated" attribute instead:
It also produces a warning (thus causing the build to fail due to
-Werror), and is at least getting close to the intention here.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -55,8 +55,12 @@
 #ifdef LIBXL_H
 # error libxl.h should be included via libxl_internal.h, not separately
 #endif
-#define LIBXL_EXTERNAL_CALLERS_ONLY \
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)
+# define LIBXL_EXTERNAL_CALLERS_ONLY \
     __attribute__((warning("may not be called from within libxl")))
+#else
+# define LIBXL_EXTERNAL_CALLERS_ONLY __attribute__((__deprecated__))
+#endif
 
 #include "libxl.h"
 #include "_paths.h"