[244] Add note on compiling auth plugin on OSX.

Closes #244.

Bug: https://github.com/eclipse/mosquitto/issues/244

src/mosquitto_plugin.h

@@ -34,6 +34,11 @@ struct mosquitto_auth_opt {
  * shared library. Using gcc this can be achieved as follows:
  *
  * gcc -I<path to mosquitto_plugin.h> -fPIC -shared plugin.c -o plugin.so
+ *
+ * On Mac OS X:
+ *
+ * gcc -I<path to mosquitto_plugin.h> -fPIC -shared plugin.c -undefined dynamic_lookup -o plugin.so
+ *
  */
 
 /* =========================================================================