Update to modern CMake style

Change-Id: I6c64edc053d65fecb7db2d6a39a38c40e7fdf9ef
Allow location of OSMesa install to be specified

Allow the location of an OSMesa install to be specified and
search there first for includes and library. The location
can be specified by setting OSMESA_ROOT.

Change-Id: I0afe52b9f81a70a990d36d6895a3ce50720706a9