Add preprocessor macros to allow compile for ios

SquiLu-ext/mpdecimal.h

@@ -1 +1,5 @@
-#include "../mpdecimal/mpdecimal.h"
+#ifdef COMPILING_FOR_IOS
+#include "../mpdecimal-ios/mpdecimal.h"
+#else
+#include "../mpdecimal/mpdecimal.h"
+#endif