Thread-safe alternative to strerror()

Thread-safe alternative to strerror()

Uses:
  - strerror_s() on Windows;
  - the GNU strerror_r() on non-Windows systems where _GNU_SOURCE is
    defined (which is currently the case when the GNU libstdc++ is used,
    even if one doesn't explicitely define _GNU_SOURCE, cf.
    <https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.predefined>);
  - the XSI-compliant strerror_r() on other systems, as long as
    _POSIX_C_SOURCE >= 200112L (otherwise, the compilation will abort
    due to a #error preprocessor instruction).