Add mechanism to flag subdirs as ignored to avoid warnings

partial backport of !3356 (merged)

added Build enhancement legacy labels

mentioned in merge request !3356 (merged)

added 1 commit

• d7a35761 - Fix compatibilty with CMake < 3.19

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added 2 commits

• abfa338b - Add mechanism to flag subdirs as ignored to avoid warnings
• 55ea9d68 - Fix compatibilty with CMake < 3.19

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added all-slots label

@clemenci Assuming the nightlies are fine can I merge this and !3425 (merged) for a new release?

• [2022-05-12 00:58] Validation started with lhcb-run2-gaudi-head#325
• [2022-05-12 00:59] Validation started with lhcb-run2-patches-dev4#1054
• [2022-05-12 01:03] Validation started with lhcb-run2-patches#1142
• [2022-05-13 00:39] Validation started with lhcb-run2-patches-dev4#1055
• [2022-05-14 09:18] Validation started with lhcb-run2-patches-dev4#1056
• [2022-05-15 00:16] Validation started with lhcb-run2-patches-dev4#1057
• [2022-05-16 00:30] Validation started with lhcb-run2-patches-dev4#1058
• [2022-05-17 00:24] Validation started with lhcb-run2-patches-dev4#1059
• [2022-05-18 00:29] Validation started with lhcb-run2-patches-dev4#1060
• [2022-05-19 00:37] Validation started with lhcb-run2-gaudi-head#326
• [2022-05-19 00:37] Validation started with lhcb-run2-patches#1143
• [2022-05-19 00:37] Validation started with lhcb-run2-patches-dev4#1061
• [2022-05-20 00:34] Validation started with lhcb-run2-patches-dev4#1062

merged

mentioned in commit 89727b4a