Running bigFORTH on amd64

I recently discovered bigFORTH and MINOS on Bernd Paysan's web site and
decided to try it on my amd64 PC running Devuan, a Debian-based Linux
distribution.

I was able to build and install it. The console REPL "bigforth" appears
to work, but when I try to run the GUI "xbigforth" it crashes with the
following message:

     libX11.so.6 Library not found!
     Level: (ABORT" @LIB @SYMS $101471E6 XOPENDIPLAY $FFE44E02 $565BE3CA $FFE440C8

(The first message is displayed in spite of the fact that package
libx11-6 is already installed.)

Any clue what I'm doing wrong?

--
David Meyer
Takarazuka, Japan
papa@sdf.org