Never ever even try to use X11 under OSX (Darwin)

Since it's done with dlsym() and libdl isn't available until 10.3, this
would require a different binary for old and newer versions of OSX.

src/conio/Common.gmake

@@ -8,6 +8,9 @@ X_PRESENT	= $(shell if [ ! \( -f $(X_HEADERS)$(DIRSEP)X11$(DIRSEP)Xlib.h -a -f $
 ifeq ($(X_PRESENT),NO)
  NO_X=1
 else
+ ifeq ($(os),darwin)	# No X11 for OSX
+  NO_X=1
+ endif
 endif
 
 ifeq ($(os),sunos)