OK. I have made some progress with this!
First, the cxref I built works, but the installed cxref (using Synaptic
Package Manager) finishes without error but generates precious little
info - just lists the file with no detail info.
Second, here is my new .cxref:
-xref-all
-index-all
-html-src
-I.
-I./lib
-I./board3d
-I./pixmaps
-I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include
-DHAVE_CONFIG_H
-DLOCALEDIR="/usr/local/share/locale"
-DAC_DOCDIR=/usr/local/share/doc/gnubg
-DAC_DATADIR=/usr/local/share
-DAC_PKGDATADIR=/usr/local/share/gnubg
-DG_DISABLE_ASSERT
-D_Float16=float
-D_Float16x=float
-D_Float32=float
-D_Float32x=float
-D_Float64=float
-D_Float64x=float
-D_Float128=float
-D_Float128x=float
Note that the -D_Float* defines are new. They probably aren't correct to
compile but they help cxref at least. Also -DLOCALEDIR uses quotes.
