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ARROW-7725: [C++] Add infrastructure for unity builds and precompiled headers

Unity builds and precompiled headers can be enabled "easily" in CMake 3.16:
https://cmake.org/cmake/help/v3.16/prop_tgt/UNITY_BUILD.html
https://cmake.org/cmake/help/v3.16/command/target_precompile_headers.html

They can make builds faster in some conditions, especially on CI with little parallelism and caching.

Some care is required to make them work:

- unity builds concatenate a number of C/C++ source files together,
  so there should be no symbol clashes, C++ namespace resolution issues
  or conflicting preprocessor definitions

- precompiled headers require that the same compile options are passed
  for all files using the header

Closes #6324 from pitrou/ARROW-7725-pch-unity-builds and squashes the following commits:

2811febac <Antoine Pitrou> ARROW-7725:  Add infrastructure for unity builds and precompiled headers

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>