diff -aur netpbm-10.73.31.old/buildtools/manpage.mk netpbm-10.73.31/buildtools/manpage.mk
--- netpbm-10.73.31.old/buildtools/manpage.mk	2020-05-28 18:41:20.400790506 +0200
+++ netpbm-10.73.31/buildtools/manpage.mk	2020-05-28 18:50:50.444143414 +0200
@@ -388,9 +388,9 @@
 # This will install the generated man pages
 installman:
 	set -x
-	for f in netpbm.1 $(MAN1); do if [ -f $$f ]; then gzip <$$f >$(MANDIR)/man1/$$f.gz; fi; done
-	for f in $(MAN3); do if [ -f $$f ]; then gzip <$$f >$(MANDIR)/man3/$$f.gz; fi; done
-	for f in $(MAN5); do if [ -f $$f ]; then gzip <$$f >$(MANDIR)/man5/$$f.gz; fi; done
+	for f in netpbm.1 $(MAN1); do if [ -f $$f ]; then gzip -n <$$f >$(MANDIR)/man1/$$f.gz; fi; done
+	for f in $(MAN3); do if [ -f $$f ]; then gzip -n <$$f >$(MANDIR)/man3/$$f.gz; fi; done
+	for f in $(MAN5); do if [ -f $$f ]; then gzip -n <$$f >$(MANDIR)/man5/$$f.gz; fi; done
 
 # This will uninstall them
 uninstallman: