Use built-in backtrace function if one exists

This gives us backtraces on panic on when you're not using GCC.

git-svn-id: http://www.varnish-cache.org/svn/trunk@4382 d4fa192b-c00b-0410-8231-f00ffab90ce4

configure.ac

@@ -213,6 +213,7 @@ AC_CHECK_FUNCS([setproctitle])
 AC_CHECK_FUNCS([srandomdev])
 AC_CHECK_FUNCS([strlcat strlcpy])
 AC_CHECK_FUNCS([strndup])
+AC_CHECK_FUNCS([backtrace])
 # white lie - we don't actually test it
 AC_MSG_CHECKING([whether daemon() works])
 case $target in

lib/libvarnishcompat/execinfo.c

@@ -29,6 +29,8 @@
 #include "svnid.h"
 SVNID("$Id: execinfo.c,v 1.3 2004/07/19 05:21:09 sobomax Exp $")
 
+#ifndef HAVE_BACKTRACE
+
 #include "compat/execinfo.h"
 
 #if defined (__GNUC__) && __GNUC__ >= 4	/* XXX Correct version to check for ? */
@@ -428,4 +430,5 @@ backtrace_symbols(void *const *buffer, int size)
 	return (0);
 }
 
-#endif
+#endif /*  (__GNUC__) && __GNUC__ >= 4 */
+#endif /* HAVE_BACKTRACE */