make sure LCG pkg-conf looks for packages in CMAKE_PREFIX_PATH and system

this is currently done in LHCb, but should be done in all projects using the LCG toolchains (as it already happens in the new lcg-toolchains).

changed milestone to %2.0.20

merged

mentioned in commit 45399bb1

mentioned in merge request lhcb/Run2Support!8 (merged)

mentioned in issue lhcb/Run2Support#1 (closed)