set QMAKE_INTERNAL_INCLUDED_FILES even without PROEVALUATOR_FULL

some qt prfs use this variable, so better set it it to avoid some noise.

Change-Id: I606c88dd7664b1cd8b490d60badd5c6bf80fd1c9
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtcreator/e64cc71194cbe283dfe9bd2cd688f01fcdcadf34)
don't turn = into += in cumulative mode

it leads to pathological cases where the number of loop iterations may
go way beyond the reasonable.

this means that users need to avoid using the = operator in alternative
branches that lead to different sources/subdirectories being included
into the project. this is a bit of a corner case anyway, as people
usually add directly to SOURCES/SUBDIRS.

Change-Id: I7783e318fbc2790f6a853ba4e3f4a12db881feb5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
(cherry picked from qtcreator/30bd7fcce1aef974f6af9eaa6532aa1f2b6192d2)
do not look for mkspecs/ directories in project trees

this pretty surprising behavior would interfere with building the
examples from an installed qt tree with qmakes from other qt builds.

.qmake.conf (and .qmake.cache) files provide a possibility to explicitly
"anchor" project roots, so there is no point in having a second, even
more magic way to do it.

Task-number: QTBUG-35485
Change-Id: I8fd4fda67cabafdf55e7a98282dcdfaffb4a405e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
(cherry picked from qtbase/65a676372c533e8b35117081ec0aa1a94e1500e5)
Reviewed-by: hjk <hjk121@nokiamail.com>