cmake: Add support for "External Binary Resource" in qt5_add_resources

Qt5 supports external binary resources (.rcc) but the cmake function qt5_add_resources cannot create it.

We tried it with the following command:
qt5_add_resources(QRC_FIXTURE "fixture.qrc" OPTIONS "--binary")

But it won't create any file. I looked into the sources and it seems that it will create a qrc_XXX.cpp everytime. But that should be a .rcc file with given "--binary" option.

So it would be a helpful feature if qt5_add_resources would support .rcc files.