Fix setenv not defined in older emscripten verions.

platform/javascript/javascript_main.cpp

@@ -33,6 +33,7 @@
 #include "platform/javascript/os_javascript.h"
 
 #include <emscripten/emscripten.h>
+#include <stdlib.h>
 
 static OS_JavaScript *os = NULL;
 static uint64_t target_ticks = 0;