do not erase userflags
--- a/configure.ac
+++ b/configure.ac
@@ -81,9 +81,9 @@ AC_ARG_ENABLE([debug],
     [enable_debug=yes])
 
 if test "x$enable_debug" = xyes; then
-    CFLAGS="-O2 -g -DDEBUG"
+    CFLAGS="-g -DDEBUG $CFLAGS"
 else
-	CFLAGS="-O2 -DNDEBUG"
+	CFLAGS="-DNDEBUG $CFLAGS"
 fi
 #AM_CONDITIONAL(ENABLE_DEBUG, test "$enable_debug" = "yes")