Actually it turns out that this only happens when openmotif is compiled
with xft support and xemacs is compiled without it, which is not very
likely. In fact, with Gentoo we don't allow that combination, and I only
stumbled upon it by circumventing the system :-/

Still it's probably better to prefix such an internal define.

--with-widgets=motif --with-dialogs=motif --with-scrollbars=motif
--with-xft=no causes a compilation failure, at least with openmotif 2.3.3.

This happens because motif also defines USE_XFT and --with-xft=no does
not define HAVE_XFT in the source code, and then assumes that USE_XFT
won't be defined either. This leads to:

x86_64-pc-linux-gnu-gcc -c -Wall -Wno-switch -Wundef -Wsign-compare
-Wno-char-subscripts -Wpacked -Wshadow -Wmissing-declarations
-Wmissing-prototypes -Wstrict-prototypes -Wdeclaration-after-statement 
-Wunused-parameter -g -O3 -fno-strict-aliasing -Wall -O2 -pipe
-march=native  -Demacs -I.
-I/var/tmp/portage/app-editors/xemacs-21.5.31/work/xemacs-21.5.31/src
-DHAVE_CONFIG_H -I/usr/X11R6/include redisplay-x.c
In file included from redisplay-x.c:33:
redisplay-xlike-inc.c: In function ‘separate_textual_runs_xft_mule’:
redisplay-xlike-inc.c:270: error: ‘XftChar16’ undeclared (first use in
this function)
redisplay-xlike-inc.c:270: error: (Each undeclared identifier is
reported only once
redisplay-xlike-inc.c:270: error: for each function it appears in.)
redisplay-xlike-inc.c:270: error: expected expression before ‘)’ token


I've applied the following fix in Gentoo:

	# Motif also defines USE_XFT so compiling with "motif -xft" fails,
	# since -xft turns off xft using HAVE_XFT and Motif enables only the
	# USE_XFT part.
	sed -i -e 's/USE_XFT/XEMACS_USE_XFT/' src/*xlike-inc* || die