build: look for backtrace_symbols() before enabling the Unix stack

walking code.

configure.ac

@@ -88,7 +88,7 @@ AC_CHECK_HEADERS(cxxabi.h)
 AC_LANG_POP(C++)
 
 dnl  Common C functions
-AC_CHECK_FUNCS(getcwd _getcwd)
+AC_CHECK_FUNCS(getcwd _getcwd backtrace_symbols)
 
 
 if test "${enable_debug}" = "yes"; then

src/base/assert.cpp

@@ -26,7 +26,7 @@ namespace lol
 
 void DumpStack()
 {
-#if defined HAVE_CXA_DEMANGLE
+#if defined HAVE_CXA_DEMANGLE && defined HAVE_BACKTRACE_SYMBOLS
     /* Get current stack frames */
     void *stack_ptrs[50];
     size_t size = backtrace(stack_ptrs, 50);