Restore header installation on Mac to always install to $includedir

We haven't fully agreed on whether headers should be in $includedir or
not for builds with frameworks, since frameworks carry their headers
inside. However, this change came too late for Qt 5.1 for us to assess
the potential impacts -- it's known to break at least the cmake files we
ship.

So restore the build to the way it was.

This is a partial revert of 6d61dfdbb74a2055438b999c6962f89cc3388eea.
Partial because the old behavior was partially broken: it did not
install headers for release builds. Now we always install (and in
debug-and-release mode, we might do it twice).

Task-number: QTBUG-32134
Change-Id: Ib84879c5a148d3717d16a7a90b2f5735fb5d80be
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
headersclean: process private modules correctly

Change-Id: I51c8ee0db31b35e1d991411fe0b03da6665e9a84
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
make requirement for -private suffix explicit

if a private module was used without the suffix, it would not add any
include paths, but the library would be still added. as long as the
includes were written as <Module/private/Header>, this would not become
visible, as the public modules would add the common include path ...
however, this soon won't be the case for mac frameworks any more. this
change makes the problem visible early on.

Change-Id: I8b1a20313ad736cb49507f07fa623e9aa812f651
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Merge branch 'release' into stable

Change-Id: I982a86e1b99e3ae629e8c5667bc7bdf9f4018b2e
Merge "Merge branch 'release' into stable" into refs/staging/stable