Fix Linux build when sys_siglist is not declared

[jmargeta]

Hi, Thanks for ITK-SNAP - the quickest image visualization and segmentation tool out there.

When building the on more recent gcc versions (e.g. gcc 11.2.0 bundled with Ubuntu 22.04), the Linux build fails with:

itksnap/src/GUI/Qt/main.cxx:74:27: error: ‘sys_siglist’ was not declared in this scope
   74 |   cerr << "ITK-SNAP: " << sys_siglist[sig] << endl;
      |                           ^~~~~~~~~~~

In the docs, the sys_siglist array is marked as non-standard, deprecated, and from glibc version > 2.32 is also no longer exported). This commit fixes the compilation by replacing the direct sys_siglist array access with calling the strsignal function.

For more info, see: https://man7.org/linux/man-pages/man3/strsignal.3.html It has been in the POSIX standard since 2008: https://pubs.opengroup.org/onlinepubs/9699919799/functions/strsignal.html

[pyushkevich]

Thank you for the fix, Jan!