Add a separate `Makefile.am' file for plugins directory

* Makefile.am (SCM_PLUGIN_SOURCES): Move to `plugins/Makefile.am'.
  (ETAGS_ARGS, nobase_dist_pkgguilesite_DATA): Update.
* configure.ac (AC_CONFIG_FILES): Add `plugins/Makefile.am'.
* plugins/Makefile.am: Add to the repository.