bug#15194: hop FTBFS

[Mark H Weaver]

Using guix from git 5d9cd70721aa8256f333f76c76cecc6593b6b293, hop fails
to build from source.  Here's the tail of the make transcript:

--8<---------------cut here---------------start------------->8---
bigloo -O2 -fsharing -Wall -wslots -L 
/tmp/nix-build-hop-2.4.0.drv-0/hop-2.4.0/lib -srfi enable-threads -srfi 
enable-avahi -cc gcc -copt "-O3  
-I/nix/store/l4389c6kqs5x339ai7rah7ywaxm97hcs-bigloo-4.0b/lib/bigloo/4.0b" 
-srfi hop-dynamic  -copt -fPIC -L /tmp/nix-build-hop-2.4.0.drv-0/hop-2.4.0/lib 
-unsafe -safee -c xml.scm -o o/xml.o

File "xml.scm", line 859, character 33481:
#         (let ((js-attr (xml-attribute-encode (xml-tilde->statement obj))))
#                        ^
*** ERROR:xml-tilde->attribute
Unbound variable -- xml-attribute-encode

File "xml.scm", line 646, character 24544:
#                   (let ((a (xml-attribute-encode attr)))
#                            ^
*** ERROR:xml-write-attribute1293
Unbound variable -- xml-attribute-encode

File "xml.scm", line 650, character 24709:
#         (display (xml-attribute-encode attr) p)))
#                  ^
*** ERROR:xml-write-attribute1293
Unbound variable -- xml-attribute-encode
3 errors occured, ending ...
make[1]: *** [o/xml.o] Error 255
make[1]: Leaving directory `/tmp/nix-build-hop-2.4.0.drv-0/hop-2.4.0/runtime'
make: *** [lib] Error 2
--8<---------------cut here---------------end--------------->8---