Add the ability for target_precompile_headers to REUSE multiple pch's
Hi, I have been recently investigating how feasible it would be to migrate a large build system over to CMake, and one of the issues I have run into which I cannot currently reflect in CMake, is the ability to reuse multiple pch files.
Library A -> A.pch
Library B -> B.pch
Executable Main -> A.pch + B.pch
Currently if you try to reuse multiple pch's, the last one specified is used instead.
Here is a sample project: test_cmake_pch.zip
Under Visual Studio, I can see that the main.vcxproj file only has force includes for the last Pch file defined (b). When executed, you will only get "Bar" instead of "Foo" and "Bar".
This has been shown to extensively reduce our build times with our current build system (from 50-70 minutes down to 5-15 minutes for incremental builds in CI, as well as major reduction for local development).