FreeTDM: Move custom backtrace code into ftdm_backtrace_walk() and helper functions.

Portability fix for uClibc and other (linux) environments that lack execinfo.h.

ftdm_backtrace_walk() and related return FTDM_NOTIMPL and print a message
if backtraces are not available in the current environment.

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

libs/freetdm/src/include/private/ftdm_core.h

@@ -104,10 +104,6 @@
 #include <sys/time.h>
 #endif
 
-#ifdef __linux__
-#include <execinfo.h>
-#endif
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>