Do not override distribution optimization and debug flags.

--- a/configure.ac
+++ b/configure.ac
@@ -14,6 +14,6 @@ CFLAGS="$CFLAGS $fuse_CFLAGS"
 
 if test "x$enable_debug" = xyes; then
-	CFLAGS="$CFLAGS -Wall -Wunused -DDEBUG -O0 -g3"
+	CFLAGS="$CFLAGS -DDEBUG"
 else
 	CFLAGS="$CFLAGS -DNDEBUG"
 fi