Fix :gui

Fixed #614 by @nuko8

Author: splhack

src/gui.c

@@ -37,7 +37,7 @@ static void gui_set_fg_color(char_u *name);
 static void gui_set_bg_color(char_u *name);
 static win_T *xy2win(int x, int y);
 
-#if defined(UNIX) && !defined(FEAT_GUI_MAC)
+#if defined(UNIX) && !(defined(FEAT_GUI_MAC) || defined(FEAT_GUI_MACVIM))
 # define MAY_FORK
 static void gui_do_fork(void);