CMake avplay: do not build the app if OpenGL or GLUT are not found

* Print a warning if avplay app can't be built.
* Allow the user to build avTranscoder library even if dependencies of
this app are not found.
* fix #94
Merge pull request #98 from cchampet/build_avplay_optionnal

CMake avplay: do not build the app if OpenGL or GLUT are not found