(svn r2830) Move CheckSwitchToEuro() to currency.[ch] and hide the truth about the custom currency behind a #define

currency.h

@@ -19,7 +19,11 @@ typedef struct {
 extern CurrencySpec _currency_specs[];
 extern const StringID _currency_string_list[];
 
+// XXX small hack, but makes the rest of the code a bit nicer to read
+#define _custom_currency (_currency_specs[23])
+
 uint GetMaskOfAllowedCurrencies(void);
 uint GetCurrentCurrencyRate(void);
+void CheckSwitchToEuro(void);
 
 #endif