Support Emacs 26.1 again

Since [1: 1ebac69] Emacs 26.1 effectively wasn't supported anymore and
[2: fa83e81] made that formal, but we should support that release at
least a while longer, which do by using the Compat package.

1: 2023-07-18 1ebac69
   package-recipe-validate-all: Report progress

2: 2023-12-24 fa83e81
   Fix CI build by requiring a newer Emacs

Makefile

@@ -3,6 +3,10 @@ include default.mk
 
 .PHONY: test
 
+# https://github.com/emacscollective/workflows/blob/main/bin/install-deps
+# expects this to find this in this file.
+DEPS  = compat
+
 all: lisp
 
 help:

default.mk

@@ -8,7 +8,7 @@ ELS  += package-build.el
 ELS  += package-recipe-mode.el
 ELCS  = $(ELS:.el=.elc)
 
-DEPS  =
+DEPS  = compat
 
 VERSION ?= $(shell test -e $(TOP).git && git describe --tags --abbrev=0 | cut -c2-)
 

package-build.el

@@ -13,7 +13,7 @@
 ;; Keywords: maint tools
 
 ;; Package-Version: 4.0.0.50-git
-;; Package-Requires: ((emacs "27.1"))
+;; Package-Requires: ((emacs "26.1") (compat "27.1"))
 
 ;; SPDX-License-Identifier: GPL-3.0-or-later