cmake: allow sourcepp libraries to be built as dlls #839

Job	Run time
build (Release, macos-latest)	3m 16s
build (Debug, ubuntu-latest)	3m 24s
build (Release, windows-latest)	7m 32s
build (Debug, macos-latest)	3m 4s
build (Debug, windows-latest)	6m 16s
build (Release, ubuntu-latest)	4m 35s
	28m 7s

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