Define __TCC_BACKTRACE_ENABLED__ if backtrace is enabled

If the `-btN` was specified, then the __TCC_BACKTRACE_ENABLED__
macro will be defined. This provides a way for user code
to tell if tcc_backtrace is defined or not.

tccpp.c

@@ -3756,6 +3756,10 @@ static void tcc_predefs(TCCState *s1, CString *cs, int is_asm)
 #ifdef CONFIG_TCC_BCHECK
     if (s1->do_bounds_check)
       putdef(cs, "__BOUNDS_CHECKING_ON");
+#endif
+#ifdef CONFIG_TCC_BACKTRACE
+    if (s1->do_backtrace)
+      putdef(cs, "__TCC_BACKTRACE_ENABLED__");
 #endif
     cstr_printf(cs, "#define __SIZEOF_POINTER__ %d\n", PTR_SIZE);
     cstr_printf(cs, "#define __SIZEOF_LONG__ %d\n", LONG_SIZE);