Apply by doing:
       cd /usr/src/gnu/usr.bin/groff/nroff
       patch -p0 < 006_nroff.patch

And then reinstall nroff.

Index: nroff.sh
===================================================================
RCS file: /cvs/src/gnu/usr.bin/groff/nroff/nroff.sh,v
retrieving revision 1.1.1.1
retrieving revision 1.2
diff -u -r1.1.1.1 -r1.2
--- nroff.sh    1996/09/14 19:01:54     1.1.1.1
+++ nroff.sh    1999/07/26 22:04:13     1.2
@@ -24,7 +24,7 @@
               echo "$prog: option $1 requires an argument" >&2
               exit 1
               ;;
-       -i|-[mrno]*)
+       -i|-S|-[mrno]*)
               opts="$opts $1";
               ;;

@@ -58,4 +58,4 @@

# This shell script is intended for use with man, so warnings are
# probably not wanted.  Also load nroff-style character definitions.
-exec groff -Wall -mtty-char $T $opts ${1+"$@"}
+exec groff -S -Wall -mtty-char $T $opts ${1+"$@"}