Remove compatibility with old cmake. #636

Job	Run time
mmg-debug / ci (ubuntu-20.04, on, off, on, off, int32_t)	1h 17m 2s
mmg-debug / ci (ubuntu-20.04, on, off, off, off, int32_t)	1h 36m 34s
mmg-debug / ci (ubuntu-20.04, off, off, on, off, int32_t)	1h 1m 14s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int32_t)	1h 11m 42s
mmg-debug / ci (macos-12, on, off, on, off, int32_t)	48m 15s
mmg-debug / ci (macos-12, on, off, off, off, int32_t)	58m 1s
mmg-debug / ci (macos-12, off, off, on, off, int32_t)	33m 46s
mmg-debug / ci (macos-12, off, off, off, off, int32_t)	49m 30s
mmg-debug / ci (ubuntu-20.04, off, off, off, on, int32_t)	5m 24s
mmg-debug / ci (macos-12, off, off, off, on, int32_t)	7m 12s
mmg-debug / ci (ubuntu-20.04, off, on, on, off, int32_t)	1h 1m 59s
mmg-debug / ci (macos-12, off, on, on, off, int32_t)	42m 38s
mmg-debug / ci (windows-2022, off, off, off, off, int32_t)	50m 47s
mmg-debug / ci (windows-2022, on, off, off, off, int32_t)	53m 7s
mmg-debug / ci (windows-2022, off, on, off, off, int32_t)	54m 50s
mmg-debug / ci (ubuntu-20.04, on, on, on, on, int64_t)	4m 47s
mmg-debug / ci (ubuntu-20.04, off, off, on, on, int64_t)	1h 7m 25s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int64_t)	2m 18s
mmg-debug / ci (macos-12, on, on, on, on, int64_t)	10m 43s
mmg-debug / ci (macos-12, off, off, off, off, int64_t)	4m 29s
mmg-debug / ci (windows-2022, on, on, off, off, int64_t)	4m 54s
mmg-debug / ci (windows-2022, off, off, off, off, int64_t)	4m 54s
mmg-debug / upload_coverage	1m 34s
	14h 33m 5s

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