Add support for modular build structure. #511

Job	Run time
B2 gcc-11	5m 2s
CMake msvc-14.2 Debug C++17	6m 21s
B2 clang-14	5m 11s
B2 msvc-14.2	7m 30s
CMake msvc-14.2 Release C++17	6m 50s
CMake gcc-11 17 Debug	10m 45s
B2 msvc-14.3	7m 27s
CMake msvc-14.3 Debug C++20	8m 55s
CMake gcc-11 20 Release	14m 26s
CMake msvc-14.3 Release C++20	9m 1s
CMake clang-11 17 Debug	7m 27s
CMake clang-11 20 Debug	7m 37s
CMake clang-13 17 Release	10m 50s
CMake clang-13 20 Release	10m 55s
CMake clang-14 17 Debug -stdlib=libc++	9m 44s
CMake clang-14 20 Release -stdlib=libc++	14m 29s
	2h 22m 30s

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