update m_featureRoots less aggressively

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>