Rearrange OSX vs. iOS defines. Disable callbacks on iOS.

src/lj_errmsg.h

@@ -162,7 +162,11 @@ ERRDEF(FFI_BADIDX,	LUA_QS " cannot be indexed")
 ERRDEF(FFI_WRCONST,	"attempt to write to constant location")
 ERRDEF(FFI_NODECL,	"missing declaration for symbol " LUA_QS)
 ERRDEF(FFI_BADCBACK,	"bad callback")
+#if LJ_OS_NOJIT
+ERRDEF(FFI_CBACKOV,	"no support for callbacks on this OS")
+#else
 ERRDEF(FFI_CBACKOV,	"too many callbacks")
+#endif
 ERRDEF(FFI_NYIPACKBIT,	"NYI: packed bit fields")
 ERRDEF(FFI_NYICALL,	"NYI: cannot call this C function (yet)")
 #endif