debian/compat: Bump to 12 (stretch-backports) and fix

debian/compat: Bump to 12 (stretch-backports) and fix

Necessary changes:

 * Delete obsolete "install" target from rules, which stopped the
   actual dh install from working.  This was broken dead code but
   seems to have actually become a proble with to the changes in v9
   for usual target dependencies.

Precautionary changes:

 * Pass INSTALL="install --strip-program=true" to make install.  This
   is as dh v11 does.  It seems like a good plan to do it here.  I
   have not examined the actual binaries etc. to check for debugging
   info.

 * Pass --no-parallel to dh.  I don't trust vtwm's 90s-era Makefile to
   necessarily be parallel-safe and vtwm builds very quickly anyway.

Closes: #965873