PoolCnv dependency generation update, 1.0 branch (2017.11.09.)
This required a bit of thinking and experimentation...
Introduced a mechanism with which _atlas_add_cnv_library(...)
can detect when it should re-generate its output files during the build. Previously the code could not detect when the generation of a converter was removed. (In a merge request for instance.)
This is to fix the issue described in ATLINFR-2018. So that merge requests would be free to add/remove EDM classes at will, and the CI system wouldn't fall over because of this.