Conditionalize use of -Wno-suggest-attribute=format pragma

Many GCC versions don't understand -Wno-suggest-attribute=format
so the pragma must only be used when supported

m4/virt-compile-warnings.m4

@@ -215,6 +215,12 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
         ;;
     esac
 
+    case $WARN_CFLAGS in
+        *-Wsuggest-attribute=format*)
+           AC_DEFINE([HAVE_SUGGEST_ATTRIBUTE_FORMAT], [1], [Whether -Wsuggest-attribute=format works])
+        ;;
+    esac
+
     # Silence certain warnings in gnulib, and use improved glibc headers
     AC_DEFINE([lint], [1],
       [Define to 1 if the compiler is checking for lint.])

src/internal.h

@@ -234,9 +234,15 @@
 #  define VIR_WARNINGS_NO_CAST_ALIGN \
     _Pragma ("GCC diagnostic push") \
     _Pragma ("GCC diagnostic ignored \"-Wcast-align\"")
-#  define VIR_WARNINGS_NO_PRINTF \
+
+#  if HAVE_SUGGEST_ATTRIBUTE_FORMAT
+#   define VIR_WARNINGS_NO_PRINTF \
     _Pragma ("GCC diagnostic push") \
     _Pragma ("GCC diagnostic ignored \"-Wsuggest-attribute=format\"")
+#  else
+#   define VIR_WARNINGS_NO_PRINTF \
+    _Pragma ("GCC diagnostic push")
+#  endif
 
 #  define VIR_WARNINGS_RESET \
     _Pragma ("GCC diagnostic pop")