Properly implement a 'make docs' target for subdirs and apps/libs

Properly implement a 'make docs' target for subdirs and apps/libs

Only call qdoc for projects which sets the QMAKE_DOCS variable to
point to a qdocconf file.

Exclude examples/ and tests/ from the qdoc run, by adding
    no_docs_target
to CONFIG for those projects.

Change-Id: Ic856c8f19db59309302d0602b3e99735609e525a
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>