Add support for modular build structure. #503

Job	Run time
B2 msvc-14.2	6m 51s
B2 gcc-11	5m 2s
B2 msvc-14.3	6m 51s
CMake gcc-11 17 Debug	10m 57s
B2 clang-14	5m 17s
CMake msvc-14.2 Debug C++17	7m 51s
CMake gcc-11 20 Release	13m 0s
CMake clang-11 17 Debug	7m 22s
CMake clang-11 20 Debug	7m 43s
CMake msvc-14.2 Release C++17	8m 37s
CMake clang-13 17 Release	10m 57s
CMake msvc-14.3 Debug C++20	7m 58s
CMake clang-13 20 Release	11m 5s
CMake msvc-14.3 Release C++20	8m 46s
CMake clang-14 17 Debug -stdlib=libc++	10m 8s
CMake clang-14 20 Release -stdlib=libc++	13m 40s
	2h 22m 5s

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