Log Message:
> When building on Interix, add Xext to X_EXTRA_LIBS.  The xditview
> program requires Xaw, which requires the Xlib shape routines, which on
> Interix are in Xext.  http://www.faqs.org/faqs/x-faq/part6/section-27.html

Move fix from Makefile to options.mk, so it is only used when the x11
option is enabled.