don't unnecessarily use QFileInfo::filePath()

just directly use the filename we constructed it from.

Change-Id: Ia428a2cb4b192fea6bde62dfbf35361bcfc4b21e
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
(cherry picked from qtcreator/571234786a3ff7e8e3a9220f12d22a9f74f7a53c)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
minor optimization: don't concatenate strings needlessly

Change-Id: Iddec1a818ff9f3ad8b12491100883e433e4b8246
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
(cherry picked from qtcreator/12652c20711fd29dcba62b8d5ba71c077d8bd06c)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
cache results of feature search

looking up the same files in the same locations over and over again
is a rather significant waste. in particular, looking up the CONFIG
flags that don't correspond with features has a measurable impact on qt
creator's project loading time.

Task-number: QTCREATORBUG-9154
Change-Id: Ibae3d8b7797e706a6416a7d45c77734ab1281b51
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
(cherry picked from qtcreator/fa27cd79e05aed4ebd16d5648480cc7d48fefd43)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
update m_featureRoots less aggressively

it's not necessary to immediately re-calculate the roots after assigning
QMAKE_PLATFORM - it's sufficient to do it on-demand, so merely
invalidate them. this avoids that we re-calculate them multiple times
without ever using them in between while processing specs with
distributed platform assignments.

Change-Id: If508594764811b96a577fc81c5ded34ab0453148
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
(cherry picked from qtcreator/28df27d924bb407791a76de8159c9ffa6efde283)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>