ci, make: Support specifying external seq

Transient 0.5.0 requires seq 2.24, which doesn't ship with Emacs until
29.1.

.github/workflows/test.yml

@@ -16,6 +16,14 @@ jobs:
       uses: purcell/setup-emacs@master
       with:
         version: ${{ matrix.emacs_version }}
+    - name: Check out seq
+      if: matrix.emacs_version == '26.1' || matrix.emacs_version == '27.2' || matrix.emacs_version == '28.2'
+      # Note: This relies on Transient's Makefile looking in this spot
+      # (relative to Transient checkout) by default.
+      run: |
+        git clone --branch=externals/seq --depth=1 \
+          https://git.savannah.gnu.org/git/emacs/elpa.git seq
+        echo SEQ_DIR=$PWD/seq >>$GITHUB_ENV
     - name: Check out compat
       uses: actions/checkout@v2
       with:

Makefile

@@ -1,10 +1,11 @@
 
 -include config.mk
 
+SEQ_DIR ?= /dev/null
 COMPAT_DIR ?= /dev/null
 TRANSIENT_DIR ?= /dev/null
 
-LOAD_PATH = -L $(COMPAT_DIR) -L $(TRANSIENT_DIR) -L .
+LOAD_PATH = -L $(SEQ_DIR) -L $(COMPAT_DIR) -L $(TRANSIENT_DIR) -L .
 EMACS = emacs
 BATCH = $(EMACS) -Q --batch $(LOAD_PATH)