Add mg_log_set_fn(), MG_ENABLE_LOG

src/log.h

@@ -8,11 +8,19 @@ void mg_log(const char *fmt, ...);
 bool mg_log_prefix(int ll, const char *file, int line, const char *fname);
 void mg_log_set(const char *spec);
 void mg_hexdump(const void *buf, size_t len);
+void mg_log_set_fn(void (*logfunc)(unsigned char ch));
 
+#if MG_ENABLE_LOG
 #define MG_LOG(level, args)                                                \
   do {                                                                     \
     if (mg_log_prefix((level), __FILE__, __LINE__, __func__)) mg_log args; \
   } while (0)
+#else
+#define MG_LOG(level, args) \
+  do {                      \
+    if (0) mg_log args;     \
+  } while (0)
+#endif
 
 #define MG_ERROR(args) MG_LOG(MG_LL_ERROR, args)
 #define MG_INFO(args) MG_LOG(MG_LL_INFO, args)