Add support for modular build structure. #510

Job	Run time
B2 msvc-14.2	2m 0s
B2 gcc-11	1m 23s
CMake msvc-14.2 Debug C++17	7m 15s
B2 msvc-14.3	1m 48s
B2 clang-14	1m 38s
CMake msvc-14.2 Release C++17	9m 39s
CMake msvc-14.3 Debug C++20	6m 17s
CMake gcc-11 17 Debug	10m 51s
CMake msvc-14.3 Release C++20	7m 40s
CMake gcc-11 20 Release	13m 22s
CMake clang-11 17 Debug	7m 32s
CMake clang-11 20 Debug	7m 37s
CMake clang-13 17 Release	10m 43s
CMake clang-13 20 Release	11m 1s
CMake clang-14 17 Debug -stdlib=libc++	9m 55s
CMake clang-14 20 Release -stdlib=libc++	13m 58s
	2h 2m 39s

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