PCH: Copy the timestamp from an absolute header file

Cristian Adamrequested to merge
cristianadam/cmake:pch-file-time into master

If target_precompile_headers contains a file from the project, then CMake will set the timestamp for cmake_pch.h|xx from that file.

This helps with ccache and precompile headers.

Fixes: #19923 (closed)
Backport: release

Edited by Brad King

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