Disable unnecessary stuff if not main project

[LucaCiucci]

I had some problems while including cereal in my project using FetchContent, so I made some small modifications to the CMakeLists.txt:

• renamed some options using the CEREAL_ prefix to make clear to which project they belong to and avoid conflicts
• changed the default value of some options so that, when not the main project, doc, sandbox and performance comparisons are disabled by default
• added a more complete check for main project

I think this edits would make it easier to depend on cereal using FetchContent, including it as submodule or similar techniques. See this other pull request for more info.

[LucaCiucci]

I see that some CI checks are not successful, but the errors are unrelated to this particular PR, am I right?

[AzothAmmo]

Yes CI seems to be borked - was hoping to go through and slam through a bunch of these PRs but will have to fix that before I do so.