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>

configure.in

@@ -495,6 +495,13 @@ AC_CHECK_FUNCS([gethostname vasprintf mmap mlock mlockall usleep getifaddrs time
 AC_CHECK_FUNCS([sched_setscheduler setpriority setrlimit setgroups initgroups])
 AC_CHECK_FUNCS([wcsncmp setgroups asprintf setenv pselect gettimeofday localtime_r gmtime_r strcasecmp stricmp _stricmp])
 
+# Check availability and return type of strerror_r
+# (NOTE: apr-1-config sets -D_GNU_SOURCE at build-time, need to run the check with it too)
+save_CPPFLAGS="$CPPFLAGS"
+CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
+AC_FUNC_STRERROR_R
+CPPFLAGS="$save_CPPFLAGS"
+
 AX_HAVE_CPU_SET
 
 AC_CHECK_LIB(rt, clock_gettime, [AC_DEFINE(HAVE_CLOCK_GETTIME, 1, [Define if you have clock_gettime()])])