This works around a breakage introduced in 2.2.21.
Hopefully the patch can be removed for 2.2.22.

--- doc/Makefile.in.orig	2020-07-09 13:22:35 UTC
+++ doc/Makefile.in
@@ -1235,8 +1235,8 @@ defsincdate: $(gnupg_TEXINFOS)
 	if test -e $(top_srcdir)/.git; then \
 	  (cd $(srcdir) && git log -1 --format='%ct' \
                -- $(gnupg_TEXINFOS) 2>/dev/null) >>defsincdate; \
-        elif test x"$SOURCE_DATE_EPOCH" != x; then   \
-	   echo "$SOURCE_DATE_EPOCH" >>defsincdate ; \
+        elif test x"$$SOURCE_DATE_EPOCH" != x; then   \
+	   echo "$$SOURCE_DATE_EPOCH" >>defsincdate ; \
 	fi
 
 defs.inc : defsincdate Makefile mkdefsinc