Fix incompatibility with Ubuntu 20.04

[srogatch]

Without this fix, the following error occurs on Ubuntu 20.04 with cmake 3.21.2:
Could NOT find OpenGL (missing: OPENGL_glx_LIBRARY)
See details in libigl/libigl#1731

[ingowald]

Hey, Serge - thanks for that; I'll have to give it a closer look first because the "build your own glfw" fall back so far was only intended for Windows, so I've never even tried that under linux.

The "easier" solution you could do under linux is simply install the libglfw3-dev package, which not only pulls in all the other dependencies you listed above, and then just uses the system-glfw instead of building one on the fly (the "build on the fly" was mostly intended for windows, where installing library like glfw is .... well.... aeh.... well). That's also why this bug was never found, previously - apparently everybody under linux already had glfw installed, so the above doesn't trigger :-)

That said, I totally agree that owl should build properly without system-glfw, and the above looks like a perfect fix (I'll just have to make sure it also runs well on windows and other platforms). So thanks!

[srogatch]

Hi Ingo! Thanks for looking into this. I think sudo apt-get install libglfw3-dev then should be among the build steps for OWL in the README.md file.