build: Support building with precompiled headers
Pre-compiled headers can save some compilation time, and we do include quite a lot of them, so let's add an unique header to be used to speedup compilation.
This is WIP as I can't figure out why this doesn't work...
g++ is called properly and the private build dir is both included and contains the gjs_pch.hh.gch
file, but for some reason it doesn't get loaded.
A simple project (pch-project.tar.xz) simulating gjs seems to work... So, no clue what's wrong [EDIT: fixed in meson].