For multi configuration generators remove per-config
qrc_FOO_$<CONFIG>.cpp source file support.
Instead use a single source file qrc_FOO.cpp which is a wrapper
that includes the actual rcc generated qrc_FOO_CONFIG.cpp file.

This way, after a repeated configuration change, only the wrapper file
qrc_FOO.cpp must be regenerated to include the appropriate
qrc_FOO_CONFIG.cpp file.