added FindOpenGL & updated OPENAL_FOUND interface

Since emscripten hides the system cmake `FindOpenGL.cmake` file, I was getting an error when trying to `find_package(OpenGL REQUIRED)`. By creating this stub (in the same style as `FindOpenAL.cmake`), I no longer had to treat Emscripten as a special case in my build script that uses OpenGL.
This is already documented two lines below: "OPENAL_FOUND, if false, do not try to link to OpenAL"?