refactor(build): simplify PCH usage
Description
Precompiled headers were introduced years ago to speed-up builds. They are active, by default.
However, our usage suffers from two issues, when switching branches:
- they may suddenly become invalid and they are not rebuilt automatically as they should
- ccache sometimes behaves weirdly and compiles with the wrong version of the PCH
Because of these two issues, we do not build with PCH enabled on Linux on our CI.
Functional specifications
If we do put any dependencies on Sight sources in the PCH, we should drastically reduce these two issues. However, it might be tough to keep the same build time, but we should try to approximate it.
Technical specifications
Use only one PCH with only external dependencies.
Test plan
CI and some manual testing... Try to switch between branches with some differences in "libs/core/*" and see if issues still appear.