I found a Japanese note about this in the developer list,

  http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-dev/22418

but even with the help of Excite's english translation, I wasn't able to tell
if patch shown at the bottom was supposed to fix the problem for HP-UX 11.

It does not seem to for HP-UX 10,

  % uname -srm
  HP-UX B.10.20 9000/782

  make[1]: Entering directory `/home/kleb/local/src/ruby-1.8.1/ext/curses'
  gcc -fPIC -g -O2  -I. -I~/local/src/ruby-1.8.1 -I~/local/src/ruby-1.8.1 -I~/local/src/ruby-1.8.1/ext/curses -DHAVE_CURSES_COLR_CURSES_H -DHAVE_BEEP -DHAVE_BKGD -DHAVE_BKGDSET -DHAVE_CURS_SET -DHAVE_DELETELN -DHAVE_DOUPDATE -DHAVE_FLASH -DHAVE_GETBKGD 
-DHAVE_GETNSTR -DHAVE_INIT -DHAVE_ISENDWIN -DHAVE_KEYNAME -DHAVE_KEYPAD -DHAVE_RESIZETERM -DHAVE_SCRL -DHAVE_SET -DHAVE_SETSCRREG -DHAVE_UNGETCH -DHAVE_WATTROFF -DHAVE_WATTRON -DHAVE_WATTRSET -DHAVE_WBKGD -DHAVE_WBKGDSET -DHAVE_WDELETELN -DHAVE_WGETNSTR 
-DHAVE_WRESIZE -DHAVE_WSCRL -DHAVE_WSETSCRREG -DHAVE_DEF_PROG_MODE -DHAVE_RESET_PROG_MODE -DHAVE_TIMEOUT -DHAVE_WTIMEOUT -DHAVE_NODELAY -DHAVE_INIT_COLOR  -c curses.c
cc1: warning: -g is only supported when using GAS on this processor,
cc1: warning: -g option disabled.
ld -b -E  -L"~/local/src/ruby-1.8.1" -L"~/local/lib" -o curses.sl curses.o  -lcur_colr -ltinfo  -ldld -lcrypt -lm   -lc
ld: Can't find library for -ltinfo
make[1]: *** [curses.sl] Error 1

~/local/src/ruby-1.8.1/ext/curses/mkmf.log has

have_library: checking for tgetent() in -ltinfo... -------------------- yes

"gcc -o conftest -I/home/kleb/local/src/ruby-1.8.1 -I/home/kleb/local/src/ruby-1.8.1  -g -O2 co
nftest.c  -L"/home/kleb/local/src/ruby-1.8.1" -L"/home/kleb/local/lib"    -E    -lruby-static -
ltinfo  -ldld -lcrypt -lm   -lc"
gcc: -lruby-static: linker input file unused because linking not done
gcc: -ltinfo: linker input file unused because linking not done
gcc: -ldld: linker input file unused because linking not done
gcc: -lcrypt: linker input file unused because linking not done
gcc: -lm: linker input file unused because linking not done
gcc: -lc: linker input file unused because linking not done
checked program was:
/* begin */

/*top*/
int main() { return 0; }
int t() { tgetent(); return 0; }
/* end */

Thanks in advance,
-- 
Bil, NASA, Hampton, Virginia, USA