Original commit message from CVS:
- GST_CHECK_MPEG2DEC macro - checks for mpeg2dec in /usr/lib.
If not found then checks for mpeg2dec in /usr/X11R6/lib.
This macro is needed because mpeg2dec depends on X11R6 libraries
and as FHS states, it should be placed in /usr/X11R6 tree but
some distros do not care and put mpeg2dec into /usr/lib.
It depends on CHECK_LIBHEADER macro.
