ref_soft: check if offsetof was defined before

ref_soft/r_local.h

@@ -35,7 +35,10 @@ typedef struct mip_s mip_t;
 
 typedef	int	fixed8_t;
 typedef	int	fixed16_t;
+
+#ifndef offsetof
 #define offsetof(s,m)       (size_t)&(((s *)0)->m)
+#endif // offsetof
 
 #define ASSERT(x) if(!( x )) gEngfuncs.Host_Error( "assert failed at %s:%i\n", __FILE__, __LINE__ )
 #define Assert(x) if(!( x )) gEngfuncs.Host_Error( "assert failed at %s:%i\n", __FILE__, __LINE__ )