CMake: Use root project identification and make proper options for install + shared/static libs

[capnkenny]

The issue found was that when using GLM via FetchContent_Declare, if different paths were specified in the call (i.e. src path, build path, etc). this would cause GLM to not properly install and would become invalid to projects attempting to use GLM in a non-header only fashion.

This PR accomplishes three things -

1. This allows for determining if GLM is the root project or not.

• If it is, it will cause GLM to produce a standalone install. This is the same behaviour master branch has today.
• If not, it will only install if GLM_INSTALL is defined as ON. Similar to today's master branch, however it gives end-users the ability to still install the project.

2. This allows for determining specifically if GLM will build as static or shared (or neither).

3. This allows for the static/shared versions of GLM to be installed.

[capnkenny]

Please do not merge until confirming with me? Looking into something for NovelRT's sake
UPDATE: Only merge if we want to require having GLM built as a static/shared library. If we are opting to go the header-only route, this is not required at all.