Fix Emacs 26 compatibility

* lisp/oc-biblatex.el (org-cite-biblatex-export-citation): Do not use
`seq-first'.
* lisp/org-persist.el (org-persist-collection-let): Suppress warnings.
* lisp/ox-latex.el (org-latex-src--engrave-mathescape-p): Do not use
`if-let'.
(org-latex-src--engrave-code): Do not use `string-trim-right'.
* lisp/ox.el (org-export-link-localise): Add require for subr-x for
`thread-first'.

lisp/oc-biblatex.el

@@ -363,7 +363,7 @@ INFO is the export state, as a property list."
           (_ nil))))
     (apply
      #'org-cite-biblatex--command citation info
-     (pcase (seq-first candidates)
+     (pcase (seq-elt candidates 0) ;; `seq-first' is not available in Emacs 26.
        (`(,_ ,_ . ,command-parameters) command-parameters)
        ('nil
         (user-error

lisp/org-persist.el

@@ -358,15 +358,14 @@ FORMAT and ARGS are passed to `message'."
   "Bind container and associated from COLLECTION and execute BODY."
   (declare (debug (form body)) (indent 1))
   `(with-no-warnings
-     ;; FIXME: We only need to suppress warnings about unused
-     ;; let-bindings.  However, it is unclear how to achieve it with
-     ;; `with-suppressed-warnings'.
      (let* ((container (plist-get ,collection :container))
             (associated (plist-get ,collection :associated))
             (path (plist-get associated :file))
             (inode (plist-get associated :inode))
             (hash (plist-get associated :hash))
             (key (plist-get associated :key)))
+       ;; Suppress "unused variable" warnings.
+       (ignore container associated path inode hash key)
        ,@body)))
 
 (defun org-persist--find-index (collection)

lisp/ox-latex.el

@@ -3420,10 +3420,10 @@ and FLOAT are extracted from SRC-BLOCK and INFO in `org-latex-src-block'."
                          (equal '("false") (cdr opt)))
                      'no)))))
             opts))))
-    (if-let ((mathescape (or (funcall mathescape-status default-options)
-                             (funcall mathescape-status options))))
-        (when (eq mathescape 'yes)
-          (or engrave-faces-latex-mathescape t)))))
+    (let ((mathescape (or (funcall mathescape-status default-options)
+                          (funcall mathescape-status options))))
+      (when (eq mathescape 'yes)
+        (or engrave-faces-latex-mathescape t)))))
 
 (defun org-latex-src--engrave-code (content lang &optional theme options inline)
   "Engrave CONTENT to LaTeX in a LANG-mode buffer, and give the result.
@@ -3443,7 +3443,7 @@ to the Verbatim environment or Verb command."
                 engrave-faces-current-preset-style))
              (engraved-buffer
               (with-temp-buffer
-                (insert (string-trim-right content "\n"))
+                (insert (replace-regexp-in-string "\n\\'" "" content))
                 (when lang-mode
                   (if (functionp lang-mode)
                       (funcall lang-mode)

lisp/ox.el

@@ -4520,6 +4520,7 @@ Return value can be an object or an element:
         (org-persist-register location-type path
                               :write-immediately t))))
 
+(require 'subr-x) ;; FIXME: For `thread-first' in Emacs 26.
 (defun org-export-link-localise (link)
   "Convert remote LINK to local link.
 If LINK refers to a remote resource, modify it to point to a local