set/use explicit QT5BUILD option...
set/use explicit QT5BUILD option...

instead of relying solely on Qt5Core_FOUND.
If set OFF/FALSE, do Qt4 build.

tzapzoor/marble,tzapzoor/marble,tzapzoor/marble,tzapzoor/marble,tzapzoor/marble,tzapzoor/marble,tzapzoor/marble,tzapzoor/marble