Add support for modular build structure. #152

Job	Run time
posix-cmake-subdir (ubuntu-22.04, OFF, 0)	54s
posix (gcc-11, 2a, ubuntu-22.04)	3m 48s
posix-cmake-install (ubuntu-22.04, OFF, 0)	40s
posix (gcc-12, 20,2b, ubuntu-22.04, g++-12)	7m 40s
posix-cmake-test (ubuntu-22.04, OFF, 0)	3m 45s
posix-cmake-install (ubuntu-22.04, ON, 0)	46s
posix-cmake-subdir (ubuntu-22.04, ON, 0)	51s
posix (gcc-13, 20,2b, ubuntu-latest, ubuntu:23.04, g++-13)	7m 52s
posix-cmake-test (ubuntu-22.04, ON, 0)	3m 31s
posix-cmake-subdir (macos-13, OFF, 1)	1m 13s
posix-cmake-install (macos-13, OFF, 1)	1m 26s
posix-cmake-test (macos-13, OFF, 1)	4m 47s
posix (clang, clang++-14, 20,2b, libc++, ubuntu-22.04, clang-14 libc++-14-dev)	7m 5s
posix-cmake-subdir (macos-13, ON, 1)	1m 25s
posix-cmake-install (macos-13, ON, 1)	1m 39s
posix-cmake-test (macos-13, ON, 1)	5m 44s
posix (clang, 20,2b, macos-13, libc++, boost-container)	9m 20s
windows (msvc-14.3, 20,latest, 32,64, windows-2022)	17m 24s
windows-cmake-install (windows-2022, OFF)	2m 49s
windows (gcc, 2a, 64, windows-2019)	2m 15s
windows-cmake-test (windows-2022, OFF)	3m 57s
windows-cmake-install (windows-2022, ON)	2m 40s
windows-cmake-subdir (windows-2022, OFF)	2m 6s
windows-cmake-test (windows-2022, ON)	4m 35s
	1h 38m 12s

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