fix-gnulib: Handle MOSTLYCLEANDIRS.

Usage of this variable suffers from similar problems as xxxCLEANFILES,
so fix it up in the same way by moving it to the end of the makefile
fragment.  The check needs to be improved a bit as the fragment contains
actual references to this variable, and only want to move the
assignments.

The variable also needs to be set before use, which strangely doesn't
actually happen in the fragment, even though this is a totally internal
thing...