[TODO] Build system improvements

[zach2good]

• [x] I have searched existing issues (https://github.com/topaz-next/topaz/issues) to see if the issue has already been opened
• [x] I have checked the commit log to see if the issue has been resolved since my server was last updated
• [x] This issue occurs on release branch (if other branch, please specify)

Additional Information (Steps to reproduce/Expected behavior) : It's been maybe 6 months since I did the grand CMake refactoring, and I haven't really looked at the build since. There are some bits left over that I just couldn't be bothered to handle at the time.

TODO:

• Audit Windows flags; https://devblogs.microsoft.com/cppblog/recommendations-to-speed-c-builds-in-visual-studio/
• Audit clang & GCC flags; https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
• Indroduce additional build flavours (-Ofast and other destructive settings etc.)
• Check that the OSX build still.. exists...
• De-globalize the remaining FindLib.cmake modules
• Have the option to build all deps from source, and have a separate repo with all our deps
• Move detour's source OUT
• Build time benchmarking
• (Re-)introduce ccache and other caching mechanisms
• PCH some things

Things to consider: fast-math will destroy everything