OPL: add the faster YMFM cores

This refactors the OPL interface in two drivers : Nuked and YMFM
Nuked is used by default, YMFM can be enabled with [Sound] fm_driver = ymfm

src/include/86box/timer.h

@@ -56,6 +56,10 @@ typedef struct pc_timer_t
     struct	pc_timer_t *prev, *next;
 } pc_timer_t;
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*Timestamp of nearest enabled timer. CPU emulation must call timer_process()
   when TSC matches or exceeds this.*/
 extern uint32_t	timer_target;
@@ -237,4 +241,8 @@ timer_process_inline(void)
     timer_target = timer_head->ts.ts32.integer;
 }
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /*_TIMER_H_*/