a note about strerror_r

[Bruno Haible]

Eric Blake wrote:
> strerror_r has its own set of portability problems, since the glibc
> signature disagrees with the POSIX signature.

glibc also has a second function __xpg_strerror_r, which it enables under
the name strerror_r when _POSIX_C_SOURCE is defined to 200112L. But this
function is also not POSIX compliant, as I've just reported:
  <http://sources.redhat.com/bugzilla/show_bug.cgi?id=12204>

Bruno