Plugin Makefile.am improvements.

Plugin Makefile.am improvements.

These were backported from recent improvements to x11/Makefile.am.

  + Use EXEEXT where appropriate.
  + Add MOSTLYCLEANFILES and mostlyclean-local.
  + Use TAGS_FILES and punt the ETAGS_ARGS misuse.
  + Add -adapter.lo to _shim_la_OBJECTS, not _shim_la_LIBADD.
  + Eliminate trailing whitespace.
  + Correct the copyright notices for x11 and x11-screen.
  + Punt stamp-scheme's dependence on -const.bin (redundant to the
    dependence on stamp-shim).