Cleanups to various hjeader and especiallyt defines in Defaults_CompilerAndStdLib_.h
fixed qCompilerAndStdLib_isnan define (sometimes two dashes)
more tweaks of qCompilerAndStdLib_isnan - still needed for msvc
isnan() is not a memory of Math namespace, but of std namespace
errno_t still not defined for gcc (not sure it ever will be - unclear a bug)
make so compiles with msvc 12 using std::thread
disable use of qUseThreads_WindowsNative set to zero when std::thread() is available; still not working