B-2: How can I build a Mule which works under the X Window System?

You can do so by defining the macro name HAVE_X_WINDOWS in $MULE/src/config.h. If you also define HAVE_X_SELECTION, you will be able to cut&paste with SELECTION of X.

But note that Mule is built by static linking, so it requires libX11.a. If you have not the library for static linking, make it by adding

  #define  ForceNormalLib YES

when you build X.