Treat warnings as errors when checking if a GCC attribute exists. We have to do this as GCC will just ignore the attribute and pop up a warning, it won't actually fail to compile.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@115276 65c4cc65-6c06-0410-ace0-fbb531ad65f3

acinclude.m4

@@ -3,13 +3,17 @@
 AC_DEFUN([AST_GCC_ATTRIBUTE],
 [
 AC_MSG_CHECKING(for compiler 'attribute $1' support)
+saved_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
 AC_COMPILE_IFELSE(
 	AC_LANG_PROGRAM([static int __attribute__(($1)) test(void) {}],
 			[]),
 	AC_MSG_RESULT(yes)
 	AC_DEFINE_UNQUOTED([HAVE_ATTRIBUTE_$1], 1, [Define to 1 if your GCC C compiler supports the '$1' attribute.]),
 	AC_MSG_RESULT(no))
-])
+]
+CFLAGS="$saved_CFLAGS"
+)
 
 # AST_EXT_LIB_SETUP([package symbol name], [package friendly name], [package option name], [additional help text])