Illegal option 's'.

diff -ur a/doc/makefile.in b/doc/makefile.in
--- a/doc/makefile.in	2022-12-01 12:16:24.110528979 -0600
+++ b/doc/makefile.in	2022-12-01 12:41:45.384526329 -0600
@@ -129,14 +129,7 @@
 version.txi: stamp.vti
 
 stamp.vti: gambit.txi $(rootfromhere)/configure.ac
-	# we need this mess because "date" and "stat"
-	# don't behave the same on Linux and BSD
-	st_mtime="`stat -s gambit.txi | sed -e 's/.*st_mtime=//g' -e 's/ .*//g'`"; \
-	if [ "$$st_mtime" = "" ]; then \
-	  mtime="`date -r gambit.txi '+%B %d, %Y'`"; \
-	else \
-	  mtime="`date -r $$st_mtime '+%B %d, %Y'`"; \
-	fi; \
+	mtime="`date -r gambit.txi '+%B %d, %Y'`";
 	echo "@set EDITION $(PACKAGE_VERSION)" > version.tmp; \
 	echo "@set UPDATED $$mtime" >> version.tmp; \
 	echo "@set VERSION $(PACKAGE_VERSION)" >> version.tmp