centralize/unify/sanitize INCLUDEPATH "enrichment"

instead of having each generator do its own magic (little surprisingly,
with different outcomes), add "stuff" to the search path in one place
used by all generators. this has several consequences:
- (unless disabled via CONFIG+=no_include_pwd) $$PWD is now consistently
  prepended by all generators. most notably, this was entirely missing
  from the MSVC generators (both nmake and VS) - despite them needing it
  most. this also affects Xcode projects.
- $$OUT_PWD (if different from $$PWD) is now added right after $$PWD,
  not at the end. this precedence clarification only makes sense, given
  that qmake tries to make shadow builds as transparent as possible.
- the qmakespec's dir is now consistently appended. the UNIX and PBX
  generators prepended it, while the rest already appended. few files
  actually include qplatformdefs.h, so having it late in the search path
  seems reasonable.
- the effect of CONFIG+=depend_includepath is now fully consistent with
  the actual include path.

Change-Id: I5f7570183351ade29342ea74fef706a0738842bf
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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