Removed custom QT5_INSTALL_PREFIX variable.

We now use the recommended approach of using CMAKE_PREFIX_PATH
for pointing to Qt5 installations. Passing this variable to
all external projects is a good idea any way (not just for Qt5).

Fixes #478.
Merge branch '478-use-cmake-prefix-path-instead-of-qt5-install-prefix'

* 478-use-cmake-prefix-path-instead-of-qt5-install-prefix:
  Removed custom QT5_INSTALL_PREFIX variable.