make use can define LIBUS_RECV_BUFFER_LENGTH length (#244)

Author: gubaojian

src/libusockets.h

@@ -19,7 +19,10 @@
 #define LIBUSOCKETS_H
 
 /* 512kb shared receive buffer */
+#ifndef LIBUS_RECV_BUFFER_LENGTH
 #define LIBUS_RECV_BUFFER_LENGTH 524288
+#endif
+
 /* A timeout granularity of 4 seconds means give or take 4 seconds from set timeout */
 #define LIBUS_TIMEOUT_GRANULARITY 4
 /* 32 byte padding of receive buffer ends */