.gitignore: Add m4 files

Existing m4/ax_check_compile_flag.m4 is skipped from the list of ignored
files.

Change-Id: I7ad810e720ee8a02a28c601aa966ddde04023819
diff --git a/.gitignore b/.gitignore
index 5f6aee2..7879088 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,8 @@
 #configure
 aclocal.m4
 autom4te.cache/
+m4/*.m4
+!m4/ax_*.m4
 config.log
 config.status
 config.guess