autotools: Only use -Wcast-align with gcc

-Wcast-align currently breaks compilation with clang.
There's a more details about it on the ell mailing list.

acinclude.m4

@@ -22,6 +22,8 @@ AC_DEFUN([COMPILER_FLAGS], [
 		CFLAGS+=" -Wdeclaration-after-statement"
 		CFLAGS+=" -Wmissing-declarations"
 		CFLAGS+=" -Wredundant-decls"
-		CFLAGS+=" -Wcast-align"
+		if ( $CC -v 2>/dev/null | grep "gcc version" ); then
+			CFLAGS+=" -Wcast-align"
+		fi
 	fi
 ])