We have to run `xpidl', but we need to provide it with the right IDL
directory.  With GNU IceCat 3 (and possibly Firefox 3), this happens to be
"$(pkg-config --variable=idldir libxul)/stable".

--- mplayerplug-in-3.55/Makefile.in	2008-01-03 01:20:33.000000000 +0100
+++ mplayerplug-in-3.55/Makefile.in	2008-09-29 22:47:20.000000000 +0200
@@ -46,7 +46,7 @@ datadir=@datadir@
 LOCALEDIR=$(datadir)/locale
 MOZILLA_HOME=@MOZILLA_HOME@
 GECKO_XPIDL=@GECKO_XPIDL@
-GECKO_IDLDIR=@GECKO_IDLDIR@
+GECKO_IDLDIR=@GECKO_IDLDIR@/stable
 
 PIXMAPS=pixmaps/logo.xpm pixmaps/progress_left.xpm pixmaps/progress_right.xpm \
 	pixmaps/progress_fill.xpm pixmaps/progress_middle.xpm