Migrate GLM to Git submodules

[feliwir]

Removing the version suffix since that is implicitly specified by the git tag - making updating simpler

[autious]

I think for now I'm going to wait with merging work that adds submodules until we've removed the Auxiliary and Proprietary submodules, and instead set up the build system to explicitally download them through some other means. Reason being that i want the tutorial we wrote to stay true.

I suspect that submoduling like this also will break downloading and compiling via "Download as zip" workflow. Making our quick-setup video obsolete.

It's important for me to maintain the simplicity of getting things up and running for budding and completely green programmers who just want to get a taste of what it might be like to dig through some code.

[feliwir]

It will break the "Download as zip" indeed and it makes sense till the private/proprietary submodules are removed.

Overall git submodules will increase the "complexity" of how to get things running, but i think there needs to be a tradeoff between being friendly to newcomers & being in a good shape for programmers who want to do more than just "compiling" the binaries.

As discussed in #15 there are many benefits of having an easy upgrade path for thirdparty dependencies and not having any partially customized external libraries.