Include and use GNUInstallDirs in cmake

[pclouds]

This installs libfastgltf.a in the expected directory. On many Linux distros this will be /usr/lib64, not /usr/lib. And this is important when the distro supports both 32 and 64 bits (32 libs go to /usr/lib)

[spnda]

First of all sorry for the late response. Do you know if this has any impact on things like vcpkg or conan? It just feels a little weird it being called GNUInstallDirs. Otherwise I'd be fine with this.

[pclouds]

I don't even have Windows so vckpg is out of question for me. Some vcpkg ticket (microsoft/vcpkg#156) does mention GNUInstallDirs though so I don't think it will cause any problem. GNUInstallDris was made working with conan too, it seems in conan-io/conan#14733.

I think this will only affect the installation phase, so if people build and use in-tree it won't have any impact whatsoever. Otherwise it will only define some variables to follow GNU Coding Standards which is the "GNU" part in GNUInstallDirs

[spnda]

Ok sure, I think I'll just merge this and see if anybody complains. I think most of the time the folders would resolve to the same as before anyway.