(build) make using zlib optional, with the caveat that some http and websocket features are not available when zlib is absent

ixwebsocket/IXHttpClient.h

@@ -90,7 +90,9 @@ namespace ix
     private:
         void log(const std::string& msg, HttpRequestArgsPtr args);
 
+#ifdef IXWEBSOCKET_USE_ZLIB
         bool gzipInflate(const std::string& in, std::string& out);
+#endif
 
         // Async API background thread runner
         void run();