Qt5 configure doesn't find the libpng built by the superbuild, but the one on the system

This matters if building on an old system like the tomviz dashboard docker container. It seems to be getting headers from one and libraries from the other:

.obj/qpnghandler.o:qpnghandler.cpp:function QPngHandlerPrivate::readPngHeader(): error: undefined reference to 'png_set_longjmp_fn'
.obj/qpnghandler.o:qpnghandler.cpp:function QPngHandlerPrivate::readPngImage(QImage*): error: undefined reference to 'png_set_longjmp_fn'