don't LoadPostFiles in build_pass reader

the two functions which use its output don't need any of the processing
done by default_post and later prf files.

Change-Id: I160a5c2762071d5dc97ed195889196b83d8e2204
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
move postprocessing of INCLUDEPATH into ProFileEvaluator::accept()

the rationale is that postprocessing which is done by the generators (in
some cases right at the point of emitting code to the makefile) should
be abstracted inside the "medium-level" class which also does the path
resolution done by the generators.

along the way we also make minor fixes to the processing:
- we pay attention to CONFIG+=no_include_pwd
- we add the build dir as well when shadow building

Change-Id: Ib389942fdc0470e05c1aa49e3615b6ac00241662
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
process QMAKE_CXXFLAGS_{APP|SHLIB|PLUGIN}

qmake helpfully hard-codes this processing in the generators instead of
letting default_pro & co. handle it, so we must explicitly reproduce it.

Task-number: <ask nikolai>
Change-Id: Ia42c727291b28cf4b01f11e464d2244ce1fb53dd
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>