Autogen: Fallback on internal qrc parser when RCC isn't built yet

When building a Qt project, the AUTORCC functionality by default uses the rcc binary to get the contents of a qrc file for dependency information. This is done at CMake "generate" time.

The problem is that while configuring Qt itself, the rcc binary is not built yet. In that case fall back to the code branch which uses an ifstream instead of the rcc binary, to get the contents of the qrc file.