Fix ROOT6 -> ROOT5 forward port

[ktf]

Should fix the mess done with #8480 and #8481 and align ROOT6 and ROOT5 builds even more.

[cmsbuild]

A new Pull Request was created by @ktf (Giulio Eulisse) for CMSSW_7_5_ROOT5_X.

Fix ROOT6 -> ROOT5 forward port

It involves the following packages:

CommonTools/Utils
CondFormats/BTauObjects
RecoBTag/PerformanceDB

@nclopezo, @cvuosalo, @cerminar, @monttj, @cmsbuild, @diguida, @slava77, @ggovi, @vadler, @mmusich can you please review it and eventually sign? Thanks.
@ferencek, @apfeiffer1, @pvmulder, @acaudron, @imarches this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
You can reject by replying to this message having '-1' in the first line of your reply.
If you are a L2 or a release manager you can ask for tests by saying 'please test' in the first line of a comment.