Find the native MPEG2 includes and library

This module defines

MPEG2_INCLUDE_DIR, path to mpeg2dec/mpeg2.h, etc.
MPEG2_LIBRARIES, the libraries required to use MPEG2.
MPEG2_FOUND, If false, do not try to use MPEG2.

also defined, but not for general use are

MPEG2_mpeg2_LIBRARY, where to find the MPEG2 library.
MPEG2_vo_LIBRARY, where to find the vo library.