Error if XvMC is requested while libmpeg2 unavailable

Edited patch to apply to master, original is here:
http://mailman.videolan.org/pipermail/vlc-devel/2009-June/061076.html

Original-by: Nicolas Chauvet
Signed-off-by: Daniel Mierswa <impulze@impulze.org>
Signed-off-by: Rafaël Carré <rafael.carre@gmail.com>

configure.ac

@@ -3584,9 +3584,13 @@ AC_ARG_ENABLE(xvmc,
 ])
 AS_IF([test "${enable_xvmc}" != "no"], [
   AC_CHECK_HEADERS(X11/extensions/vldXvMC.h, [
+    AS_IF([test "${enable_libmpeg2}" = "yes"], [
       VLC_ADD_PLUGIN([xvmc])
       VLC_ADD_LIBS([xvmc],[${X_LIBS} ${X_PRE_LIBS} -lX11 -lXext -lXvMCW -lXv ${LIBMPEG2_LIBS}])
       VLC_ADD_CPPFLAGS([xvmc],[${X_CFLAGS} ${LIBMPEG2_CFLAGS}])
+    ], [
+      AC_MSG_ERROR([XVideo-MotionCompensation needs libmpeg2 which isn't available])
+    ])
   ], [
     AC_MSG_ERROR([XVideo-MotionCompensation extension not found!])
   ])