Change where LHCB_UNUSED_SUBDIR_MESSAGE_TYPE is initialized

so that it is possible to override locally the value with a regular variable (and same for LHCB_UNUSED_SOURCE_MESSAGE_TYPE)

Closes #190 (closed)

added Build backport run2 enhancement labels

Backport to Run2 with !3425 (merged)

removed backport run2 label

added RTA label

approved this merge request

merged

mentioned in commit d87e9e13

mentioned in issue Moore#390 (closed)