make setting a bad $$[QMAKEFEATURES] not crash qmake

Task-number: QTBUG-29642
Change-Id: I9cc209eb313f03bf342bcb64b1de3005755700a7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
add default branch to switch in platformToolSetVersion()

Change-Id: Icf399ffd7e41aca207ea3b4b08b9e3dead7c821a
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
clarify intended operator precedence

Change-Id: I176bf225d4cb388f1d328ac140741fe32667e89c
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>