Add accRegisterSymbolCallback API to control external symbol linkage.

Until now dlsym was used to lookup external symbols. Now you can
register your own function to be called when an undefined symbol is
used.
diff --git a/include/acc/acc.h b/include/acc/acc.h
index e25fde0..af21a46 100644
--- a/include/acc/acc.h
+++ b/include/acc/acc.h
@@ -48,6 +48,11 @@
 
 void accDeleteScript(ACCscript* script);
 
+typedef ACCvoid* (*ACCSymbolLookupFn)(ACCvoid* pContext, const ACCchar * name);
+
+void accRegisterSymbolCallback(ACCscript* script, ACCSymbolLookupFn pFn,
+                               ACCvoid* pContext);
+
 ACCenum accGetError( ACCscript* script );
 
 void accScriptSource(ACCscript* script,