autogen.sh: paper over automake <1.14's lack of %reldir% support

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316

autogen.sh

@@ -1,2 +1,62 @@
 #!/bin/sh
-autoreconf -fiv "$(dirname "$0")" && rm -rf "$(dirname "$0")"/autom4te.cache
+[ "${0%/*}" = "$0" ] || cd "${0%/*}" || exit
+
+# %reldir%/%canon_reldir% (%D%/%C%) only appeared in automake 1.14, but RHEL/CentOS 7 has 1.13.4
+# This is an (overly) simplistic preprocessor that papers around this for the duration of the generation step,
+# and can be removed once support for CentOS 7 is dropped
+automake --version | awk '{print $NF; exit}' | (
+	IFS=. read -r AM_MAJ AM_MIN _
+	[ "$AM_MAJ" -gt 1 ] || [ "$AM_MIN" -ge 14 ]
+) || {
+	process_root() {
+		root="$1"; shift
+
+		grep -q '%[CD]%' "$root/Makefile.am" || return
+		find "$root" -name Makefile.am "$@" | while read -r dir; do
+			dir="${dir%/Makefile.am}"
+			grep -q '%[CD]%' "$dir/Makefile.am" || continue
+
+			reldir="${dir#$root}"
+			reldir="${reldir#/}"
+
+			canon_reldir="$(printf '%s' "$reldir" | tr -C 'a-zA-Z0-9@_' '_')"
+
+			reldir_slash="$reldir/"
+			canon_reldir_slash="${canon_reldir}_"
+			[ -z "$reldir" ] && reldir_slash=
+			[ -z "$reldir" ] && canon_reldir_slash=
+
+			echo "$dir/Makefile.am" >&3
+			sed -i~ -e "s:%D%/:$reldir_slash:g"       -e "s:%D%:$reldir:g" \
+			        -e "s:%C%_:$canon_reldir_slash:g" -e "s:%C%:$canon_reldir:g" "$dir/Makefile.am"
+		done 3>>"$substituted_files"
+	}
+
+	rollback() {
+		while read -r f; do
+			mv "$f~" "$f"
+		done < "$substituted_files"
+		rm -f "$substituted_files"
+	}
+
+
+	echo "Automake <1.14; papering over missing %reldir%/%canon_reldir% support" >&2
+
+	substituted_files="$(mktemp)"
+	trap rollback EXIT
+
+	roots="$(sed '/Makefile$/!d;/module/d;s:^\s*:./:;s:/Makefile::;/^\.$/d' configure.ac)"
+
+	IFS="
+"
+	for root in $roots; do
+		root="${root#./}"
+		process_root "$root"
+	done
+
+	set -f
+	# shellcheck disable=SC2086,SC2046
+	process_root . $(printf '!\n-path\n%s/*\n' $roots)
+}
+
+autoreconf -fiv && rm -rf autom4te.cache