Motif support

[Po Lu]

`--with-x-toolkit=motif' is documented in INSTALL, but configure
complained that it was not an option.  At first I thought this was
related to the PGTK merge, but apparently someone removed the single
line in configure.ac that enables the Motif build some time last year.

After adding it back, the Motif build works perfectly on both master and
emacs-28.  So I see no harm in reenabling it (possibly in the release
branch as well.)

If people are concerned about the Motif support being a maintenance
burden, then no worries: you will be free to break the Motif build
whenever you want, and I will notice and fix the lossage.