FreeSWITCH: Add switch_strerror_r() to fix problems with XSI and GNU variants of strerror_r().

GNU variant of strerror_r() returns char *, while the XSI version returns int.

To make things worse, glibc ships both and added a unused result warning
in recent versions (2.16) causing the build to fail.

Add our own custom wrapper that always returns a pointer to the message buffer
and additionally make XSI versions of strerror_r() GNU compatible by
returning "Unknown error xxx" if no error message is available.

Fixes:
    src/switch_rtp.c: In function 'rtp_common_read':
    src/switch_rtp.c:3313:15: error: ignoring return value of 'strerror_r',
    declared with attribute warn_unused_result [-Werror=unused-result]
    cc1: all warnings being treated as errors

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

src/switch_utils.c

@@ -2968,6 +2968,31 @@ SWITCH_DECLARE(unsigned long) switch_atoul(const char *nptr)
 	else return (unsigned long) tmp;
 }
 
+
+SWITCH_DECLARE(char *) switch_strerror_r(int errnum, char *buf, switch_size_t buflen)
+{
+#ifdef HAVE_STRERROR_R
+#ifdef STRERROR_R_CHAR_P
+	/* GNU variant returning char *, avoids warn-unused-result error */
+	return strerror_r(errnum, buf, buflen);
+#else
+	/*
+	 * XSI variant returning int, with GNU compatible error string,
+	 * if no message could be found
+	 */
+	if (strerror_r(errnum, buf, buflen)) {
+		switch_snprintf(buf, buflen, "Unknown error %d", errnum);
+	}
+	return buf;
+#endif /* STRERROR_R_CHAR_P */
+#else
+	/* Fallback, copy string into private buffer */
+	switch_copy_string(buf, strerror(errnum), buflen);
+	return buf;
+#endif
+}
+
+
 /* For Emacs:
  * Local Variables:
  * mode:c