If QT_QMAKE_EXECUTABLE is not set, the dashboard script asks for qt4 as default. There should be an option for qt5...

Easy fix: Set QT_QMAKE_EXECUTABLE, however, a proper solution would be nice.

Probably, the following lines in the setup script need to be changed
if(MITK_USE_QT)

if(NOT QT_QMAKE_EXECUTABLE)
  find_program(QT_QMAKE_EXECUTABLE NAMES qmake qmake-qt4
               HINTS ${QT_BINARY_DIR})
endif()