add SAASound as an alternate SAA1099 core

DOES NOT WORK YET

extern/SAASound/src/SAASound.cpp

@@ -0,0 +1,13 @@
+// Part of SAASound copyright 1998-2018 Dave Hooper <dave@beermex.com>
+//
+// SAASound.cpp - dummy function
+//
+//////////////////////////////////////////////////////////////////////
+
+#include <stdio.h>
+
+// Provide something so the compiler doesn't optimise us out of existance
+int SomeFunction ()
+{
+	return 42;
+}