Optimize build time

[brawer]

The build time for OSMExpress can be significantly reduced.

• Once Document how to build with system libs instead of vendored libs #20 is finished, the build will use pre-compiled libraries if available on the system. This will be particularly noticable with S2, which is quite a beast.
• Afterwards, change FetchContent declarations in CMakeLists.txt to fetch from ZIP archives instead of Git. This will make the initial check-out faster because it is just a single HTTP fetch per uninstalled library.

Filing this bug for myself (@brawer) so I don't forget. It's not super important, but would be nice to have.