The qtx_add_resources() macro adds the resource file to the output list
to maintain file-level dependencies. Having the qrc file in a target
sources is a precondition for AUTORCC to function.
When processing the source files of a target, only add the generated
qrc_<file>.cpp to the target sources if AUTORCC is ON. This avoids
pre-porting conflict with the macro.
Reported-by: Micha Hergarden
Stephen Kelly