[cmake] Adding support for PCH and Unity builds (using Cotire/CMake 3.16)

Have a look at https://github.com/sakra/cotire, it can speed up the build of a CMake project with low effort (by using precompiled headers).

When compiling an example like the extract_indices tutorial, precompiled headers brought down my compile time from 6s to 1.8.

For this example, with g++ -H I found that it includes over 3900 header files. A precompiled header reduces that to 1.

Note that cotire doesn't seem to be able to share precompiled headers across files so far, so it might not be able to help when doing just a single full build once, but it seems like it can dramatically improve build speeds when developing.

The speedup [doesn't seem to work with clang] right now though.

Start by reviewing the project's CMake configuration and the cotire documentation linked in the issue. Reproduce the extract_indices example build and compare precompiled-header and Unity-build behavior with g++ and clang. Done means PCH and Unity builds are supported in the CMake setup without breaking the existing example builds.