Enable special generation of xenv files to Detector and GitCondDB

The following discussion from !171 (merged) should be addressed:

• @rmatev started a discussion: (+5 comments)

  I was unfortunate enough to be using LbDevTools/master and discovered this is a problem when building Detector. More precisely, at least include/ is not added to ROOT_INCLUDE_PATH in Detector.xenv which then causes the LHCb build to fail (in genreflex).