* configure.in: support for optional gap_decode_mpeg plugin

* plug-ins/gap/Makefile.am
* plug-ins/gap/README
* plug-ins/gap/gap_arr_dialog.c
* plug-ins/gap/gap_decode_mpeg_main.c
* plug-ins/gap/gap_decode_xanim.c
* plug-ins/gap/gap_decode_xanim.h
* plug-ins/gap/gap_frontends_main.c
* plug-ins/gap/gap_main.c
* plug-ins/gap/gap_mpege.c: patches from Wolfgang Hofer

-Yosh