Manually unbreak if 0, EXTRA_DIST = (and dist_xxx_MANS =) being ignored

Workaround-for: https://bugs.debian.org/1011024
Closes: openzfs#13459
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>

autogen.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
+# shellcheck disable=SC2016,SC2094
 [ "${0%/*}" = "$0" ] || cd "${0%/*}" || exit
 
 # %reldir%/%canon_reldir% (%D%/%C%) only appeared in automake 1.14, but RHEL/CentOS 7 has 1.13.4
@@ -60,3 +61,63 @@ automake --version | awk '{print $NF; exit}' | (
 }
 
 autoreconf -fiv && rm -rf autom4te.cache
+
+# https://github.com/openzfs/zfs/issues/13459
+# https://bugs.debian.org/1011024
+# When including, EXTRA_DIST in false conditionals is skipped; we need to preprocess Makefile.in to unbreak this.
+# The same happens to dist_man_MANS (DISTFILES -> DIST_COMMON -> am__DIST_COMMON -> dist_man_MANS).
+#
+# Shortcuts taken:
+#   * only the root Makefile.in is processed because it's the only one that does if, include; it takes too long to process this as-is (~0.3s)
+#   * only one level of :suff=suff allowed
+{
+	deverbol() {
+		type rollback > /dev/null 2>&1 && rollback
+		rm -f "$verbols"
+	}
+
+	trap deverbol EXIT
+	verbols="$(mktemp)"
+
+	parse_verbols() {
+		v="${v#'$('}"
+		v="${v%')'}"
+		[ -z "$suff_bundle" ] && {
+			suff_bundle="${v#*:}"
+			[ "$suff_bundle" = "$v" ] && suff_bundle= || suff_bundle=":$suff_bundle"
+		}
+		v="${v%:*}"
+	}
+
+	printf "%s\n" dist_man_MANS EXTRA_DIST > "$verbols"
+	while read -r v suff_bundle; do
+		parse_verbols
+		# Early exit buys back 0.43s here
+		awk -v v="$v" '($0 ~ ("^(@[A-Z_@]*@)?" v " =")),!/\\$/ {copying=1; print}  copying && !/\\$/ {exit}' Makefile.in |
+			grep -o '$([^ )]*)' |
+			sed 's/$/ '"$suff_bundle"'/'
+	done < "$verbols" >> "$verbols"
+	sort "$verbols" | uniq |
+	while read -r v suff_bundle; do
+		parse_verbols
+		# And another 0.34s here
+		awk -v v="$v" '
+				BEGIN {
+					printf "faux_" v "_faux := "
+				}
+				($0 ~ ("^@[A-Z_@]*@" v " =")),!/\\$/ {
+					l = $0
+					sub(/^@[A-Z_@]*@/, "", l)
+					sub("^" v " =", "", l)
+					sub(/\\$/, "", l)
+					printf "%s", l
+					copying = 1
+				}
+				copying && !/\\$/ {
+					exit
+				}
+			' Makefile.in
+		echo
+		echo "EXTRA_DIST += \$(faux_${v}_faux${suff_bundle})"
+	done >> Makefile.in
+}