Can't compile rxvt-unicode on pkgsrc in Slackware

[Phil Thomson <pkgsrc-users%philthomson.ca@localhost>]

Hi all,

I have installed pkgsrc on a minimal install of Slackware in a virtual 
machine. When I go into /usr/pkgsrc/x11/rxvt-unicode and type 'bmake 
install clean clean-depends', I get:

[...]
command.C: In member function 'void rxvt_term::process_xterm_seq(int, 
const char*, char)':
command.C:3339:40: error: invalid conversion from 'const char*' to 
'char*'
*** Error code 1

Stop.
bmake: stopped in 
/usr/pkgsrc/x11/rxvt-unicode/work/rxvt-unicode-9.06/src
[...]

What shall I do?