Signal stack size too small on Linux

[jwalt]

On my Linux box, this line...

https://github.com/onqtam/doctest/blob/fec6e4a5f570dfa8ba4b715bc8fa4f51a8b70557/doctest/doctest.h#L4485

... expands to 8kiB of signal stack. This seems to be too small, as I sometimes get segmentation faults in logTestStart (or rather, snprintf) when handling signals. The windows branch of this code uses 32kiB, so I suggest to do the same for POSIX boxes.

Also, I suggest replacing later ocurrences of SIGSTKSZ with sizeof(altStackMem) for increased maintainability.

[onqtam]

sounds reasonable!