* [CMake] : Add precompiled header support with FLATBUFFERS_ENABLE_PCH
FLATBUFFERS_ENABLE_PCH enable precompile headers support for 'flatbuffers' and 'flatc'. Default if OFF.
You need to set it to ON to make it work. 'cmake -DFLATBUFFERS_ENABLE_PCH=ON ..'
This will only work if CMake version is >= 3.16, because target_precompile_headers is used.
If CMake doesn't have target_precompile_headers, then nothing will happen.
This can speed up compilation time.
Precompiled header file for 'flatbuffers' is 'pch.h'
Precompiled header file for 'flatc' is 'flatc_pch.h'
* Enable FLATBUFFERS_ENABLE_PCH for VC 2017 and VS2019 builds
* [CMake]: Fix error in FLATBUFFERS_ENABLE_PCH description
* Add a function add_pch_to_target to avoid copy and pasting the same code like add_fsanitize_to_target
Move pch.h and flatc_pch.h from include/flatbuffers to include/flatbuffers/pch
Make flatc_pch.h depends on pch.h since it extend it
