cmake: Fail if `Libmultiprocess` is missing when `WITH_MULTIPROCESS=ON` #3948

Job	Run time
ASan + LSan + UBSan + integer, no depends, USDT	50m 17s
test each commit	0s
Win64 native, VS 2022	2h 8m 15s
macOS 14 native, arm64, no depends, sqlite only, gui	23m 9s
Win64 native fuzz, VS 2022	45m 44s
macOS 14 native, arm64, fuzz	11m 34s
	4h 18m 59s

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