Find the native MPEG includes and library

This module defines

MPEG_INCLUDE_DIR, where to find MPEG.h, etc.
MPEG_LIBRARIES, the libraries required to use MPEG.
MPEG_FOUND, If false, do not try to use MPEG.

also defined, but not for general use are

MPEG_mpeg2_LIBRARY, where to find the MPEG library.
MPEG_vo_LIBRARY, where to find the vo library.