Add a check to configure.ac for strerror_r, since we don't currently
require POSIX.1-2001 conformance in general. Add back a
plain-strerror() case as a fallback, and rearrange the glibc-vs-POSIX
strerror_r() branches.

Update the docs to not claim that "not all platforms support the
strerror() function" (we require C90), but still mention the UTF-8 and
always-valid-string benefits. (And make test_strerror() check that
last part.)

https://bugzilla.gnome.org/show_bug.cgi?id=754788