Remove isnan() and isinf() templates from QGC.h for Windows builds

Remove isnan() and isinf() templates from QGC.h for Windows builds, this
generates errors (already defined in math.h).
Remove isnan() and isinf() templated from QGC.h only if prior to Visual Studio 2013
Describe which Qt variant to choose for a Windows build
Merge pull request #757 from Kynos/master

Remove isnan() and isinf() templates from QGC.h for Windows builds