build: Don't mind if m4 dir already exists

diff --git a/autogen.sh b/autogen.sh
index a015002..cc056c2 100755 (executable)
--- a/autogen.sh
+++ b/autogen.sh
@@ -5,7 +5,7 @@ which gnome-autogen.sh || {
     exit 1
 }
 
-mkdir m4
+mkdir -p m4
 gnome-autogen.sh --enable-vala --enable-maintainer-mode --enable-debug \
                  --enable-test-plugin --enable-mediathek-plugin \
                  --enable-gst-launch-plugin "$@"