diff --git a/sys/unix/Makefile.doc b/sys/unix/Makefile.doc
index f2f42502a..dfb812700 100644
--- a/sys/unix/Makefile.doc
+++ b/sys/unix/Makefile.doc
@@ -29,7 +29,7 @@ COLCMD ?= col -bx
 
 # The command to use to generate a PostScript file
 # PSCMD = ditroff | psdit
-PSCMD = groff
+PSCMD ?= groff
 
 # Use the "cat" GUIDECMD if nroff and/or tbl and/or col are not installed
 # Not appropriate for creating Guidebook.txt.
@@ -38,8 +38,8 @@ PSCMD = groff
 # Single page. Might need adjustment to .pl value
 # GUIDECMD= $(GUIDE_PREFORMAT) | perl -pe 's/^(.mt)$$/.pl 4720v\n.in 0\n.po 8\n.ll 64m\n$$1/' | nroff -c -Tascii | $(COLCMD)
 #
-GUIDECMD = $(GUIDE_PREFORMAT) | nroff -c -Tascii $(NROFF_FLAGS) | $(COLCMD)
-ONEPAGECMD = $(ONEPAGE_PREFORMAT) | nroff -c -Tascii $(NROFF_FLAGS) | $(COLCMD)
+GUIDECMD ?= $(GUIDE_PREFORMAT) | nroff -c -Tascii $(NROFF_FLAGS) | $(COLCMD)
+ONEPAGECMD ?= $(ONEPAGE_PREFORMAT) | nroff -c -Tascii $(NROFF_FLAGS) | $(COLCMD)
 
 # Only generate output for the current configuration:
 NHGREP = $(MAKEDEFS) --grep --input - --output -