Qt already has solutions for cross platform isnan and isinf logic. We
should use that instead of duplicating it.
This should also fix compiling tst_qstring on MinGW with C++11.
Change-Id: I7b691fd47701a8f07e1a1fe08a95a0aca43ccca1
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>