[CMake] Qt includes as SYSTEM includes

Hi!

It would be nice if the Qt5 CMake files offer an option to include the Qt headers as system headers.

CMake 2.8.11 introduced that option for Qt4:

if(QT_INCLUDE_DIRS_NO_SYSTEM)
  include_directories(${QT_INCLUDE_DIR})
else(QT_INCLUDE_DIRS_NO_SYSTEM)
  include_directories(SYSTEM ${QT_INCLUDE_DIR})
endif(QT_INCLUDE_DIRS_NO_SYSTEM)

Please also enable this for Qt5.

For instance, if we use the compiler flag '-Wold-style-cast' we get warnings inside the Qt headers which we would like to suppress that way.

Best Regards,
Gerhard