gudev: ensure autoreconf works properly

We modify the udev build system with a patch, so we need to autoreconf.
Unfortunately libtool doesn't like when the first autogen is done on one
version and autoreconf is done again with a different version.

To avoid that issue, nuke the .m4 caches so they all get rebuilt using
the same libtool version.

https://bugzilla.gnome.org/show_bug.cgi?id=670343