autogen.sh: remove all autoconf/automake caches, if any

Otherwise if you for example share root of libevent repository with some
vm (for example windows+cygwin) then if you try to build with configure
on host, and then on guest it will fail, so let's just ignore this since
autogen.sh is just for this -- cleaning all stuff.

autogen.sh

@@ -1,4 +1,7 @@
 #!/bin/sh
+
+make maintainer-clean >/dev/null 2>/dev/null
+
 if [ -x "`which autoreconf 2>/dev/null`" ] ; then
    exec autoreconf -ivf
 fi