updating CMakeLists for better robustness, when a "glad" and "glfw" are also used by other libraries

[HeCraneChen]

Hi Nick,

Thanks for the great work. These updates are fixing the situation when someone want to compile two visualizers at the same project both using "glad" and "glfw", (e.g., both libigl viewer and polyscope). It does not happen everyday, but I think better robustness is a good idea. The fixes are necessary when using this template (https://github.com/libigl/libigl-example-project) together with polyscope.

Thanks,
Crane

[nmwsharp]

I believe this is superseded by #261 which has now been merged. It makes this change as well as several other cmake changes to use proper targets. (Please reopen if this isn't covered)

Thanks for submitting this @HeCraneChen, sorry I didn't get to merging this PR directly!