Add flag making Allen dependency optional

When Moore_WITH_Allen is set to a false-y value, Moore does not depend on Allen (and some tests are disabled). The dependency is not kept optional in the find_package sense (i.e. not REQUIRED) because it might be found from an undesired location (e.g. a release on CVMFS).

This flag is useful for (some) development purposes only.

added Build enhancement labels

assigned to @lbian

Hi @rmatev! Does this MR need a ci-test?

/ci-test --merge

added ci-test-triggered label

• [2021-08-03 16:09] Validation started with lhcb-master-mr#2659

added 16 commits

• 56bad8e2...657fe1ac - 14 commits from branch master
• aeec982d - Add flag making Allen dependency optional
• bd71fd62 - Remove workaround for test timeout

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mentioned in commit 22604221

mentioned in commit MooreAnalysis@f244ad68

mentioned in merge request MooreAnalysis!44 (merged)

mentioned in commit MooreAnalysis@64aa6ed3

mentioned in commit MooreAnalysis@10ab2e87

mentioned in commit MooreAnalysis@c5308421

mentioned in issue #311 (closed)

mentioned in commit DaVinci@16136562