libdwfl: use GNU strerror_r only when available.

Some C libraries don't provide the GNU version of strerror_r, only the
XSI-compliant one. We use the GNU version when available, since it fits
the code better, and otherwise use the XSI-compliant one.

https://sourceware.org/bugzilla/show_bug.cgi?id=21010

Signed-off-by: √Črico Rolim <erico.erc@gmail.com>
4 files changed