Fix building with or without backtrace(3)

backtrace(3) and execinfo.h are GNU extensions and may or may not be
available, and they may be provided via libexecinfo.

Fix detection of libexecinfo and allow building without any support of
backtrace, in which case we let kernel create core dump.

Fixes #116 #117

tmate-debug.c

@@ -1,10 +1,19 @@
+#ifdef HAVE_EXECINFO_H
 #include <execinfo.h>
+#endif
 #include <stdio.h>
 #include <stdlib.h>
 #include <regex.h>
 #include <signal.h>
 #include "tmate.h"
 
+#ifndef HAVE_BACKTRACE
+
+void tmate_print_stack_trace(void) {}
+void tmate_catch_sigsegv(void) {}
+
+#else
+
 #if DEBUG
 
 static int print_resolved_stack_frame(const char *frame)
@@ -88,3 +97,4 @@ void tmate_catch_sigsegv(void)
 {
 	signal(SIGSEGV, handle_sigsegv);
 }
+#endif