revamp handling of qmake target mode

revamp handling of qmake target mode

the project evaluator becomes oblivious of the target mode.
the mode is set up in spec_post.prf according to the spec.
$$QMAKE_TARGET contains the feature suffixes to search, and is also
contained in $$CONFIG.
the target_mode variable itself becomes private to the Makefile class.

Change-Id: I3c06d9dab536b753343cec6c5c491d3203e50bd8
Reviewed-by: Mark Brand <mabrand@mabrand.nl>