Resolve "(build): PCH are always rebuilt with recent cmake versions"

Description

This was caused by the new CMake policy CMP0116 about the usage of the DEPFILE argument of add_custom_command. Since we did not succeed to use the NEW behaviour yet, we switch back, temporarily, to the OLD behaviour.

Closes #806 (closed)

How to test it?

• Start from a clean build, with PCH enabled (default)
• ninja core once, observe a hundred of files are built
• ninja core a second time, observe "ninja: no work to do." is returned