ox-latex: Disable precompile for xelatex/lualatex

* lisp/ox-latex.el (org-latex-make-preamble): If xelatex or
lualatex are used for LaTeX export, disable LaTeX header
precompilation (via the CTAN package mylatexformat) locally in the
buffer and issue a warning.  Xelatex does not support
precompilation, and while Lualatex supports it in some cases, it
is best left unsupported by the Org LaTeX export process until
precompilation support for them improves upstream.

* lisp/org-latex-preview.el (org-latex-preview--create-tex-file):
Make the same changes as above when precompiling LaTeX headers
for LaTeX preview purposes.

lisp/org-latex-preview.el

@@ -1510,25 +1510,30 @@ The path of the created LaTeX file is returned."
            (concat (make-temp-name "org-tex-") ".tex")
            (and remote-file-p temporary-file-directory)))
          (write-region-inhibit-fsync t)
-         (coding-system-for-write buffer-file-coding-system))
+         (coding-system-for-write buffer-file-coding-system)
+         (precompile-failed-msg))
     (when (and relative-file-p remote-file-p)
       (error "Org LaTeX Preview does not currently support \\input/\\include in remote files"))
     (when org-latex-preview-precompile
-      (if-let ((format-file (org-latex-preview--precompile processing-info header)))
-          ;; Replace header with .fmt file path.
-          (setq header (concat "%& " (file-name-sans-extension format-file)))
+      (pcase (plist-get processing-info :latex-processor)
+        ("pdflatex"
+         (if-let ((format-file (org-latex-preview--precompile processing-info header
+                                                             (not relative-file-p))))
+             (setq header (concat "%& " (file-name-sans-extension format-file)))
+           (setq precompile-failed-msg
+                 "Precompile failed.")))
+        ((or "xelatex" "lualatex")
+         (setq precompile-failed-msg
+               (concat
+                (plist-get processing-info :latex-processor)
+                " does not support precompilation."))))
+      (when precompile-failed-msg
         (display-warning
          '(org latex-preview disable-local-precompile)
-         (concat "Precompile failed, disabling LaTeX preview precompile in this buffer."
-                 "\n  To renable, run `(setq-local org-latex-preview-use-precompilation t)' or reopen this buffer."
-                 (pcase (plist-get processing-info :latex-processor)
-                   ("lualatex"
-                    "\n  LuaLaTeX is known to be problematic, if you might be able to help please get in touch with emacs-orgmode@gnu.org.")
-                   ("xelatex"
-                    ;; Note: <https://tex.stackexchange.com/questions/395965/precompile-with-xelatex-and-fontspec> might be helpful.
-                    "\n  The current XeTeX approach does not support fontspec, if you might be able to help please get in touch with emacs-orgmode@gnu.org."))
-                 "\n "))
-        (setq-local org-latex-preview-use-precompilation nil)))
+         (concat
+          precompile-failed-msg
+          " Disabling LaTeX preview precompile in this buffer.\n To re-enable, run `(setq-local org-latex-preview-precompile t)' or reopen this buffer."))
+        (setq-local org-latex-preview-precompile nil)))
     (with-temp-file tex-temp-name
       (insert header)
       ;; The \abovedisplayskip length must be set after \begin{document} because

lisp/ox-latex.el

@@ -2130,9 +2130,20 @@ specified in `org-latex-default-packages-alist' or
              "\n"))
            (format-file
             (and org-latex-precompile
-                 (org-latex--precompile
-                  info preamble
-                  (string-match-p "\\(?:\\\\input{\\|\\\\include{\\)[^/]" preamble)))))
+                 ;; Precompilation is disabled for xelatex/lualatex for now.
+                 (if (member (plist-get info :latex-compiler)
+                             '("xelatex" "lualatex"))
+                     (progn
+                       (display-warning
+                        '(org latex-export disable-local-precompile)
+                        (format "%s does not support precompilation, disabling LaTeX precompile in this buffer.\n To re-enable, run `(setq-local org-latex-precompile t)' or reopen this buffer."
+                                (plist-get info :latex-compiler)))
+                       (setf (buffer-local-value
+                              'org-latex-precompile (get-buffer (plist-get info :input-buffer)))
+                             nil))
+                   (org-latex--precompile
+                    info preamble
+                    (string-match-p "\\(?:\\\\input{\\|\\\\include{\\)[^/]" preamble))))))
       (when (and format-file (not snippet?))
         (let ((preamble-parts (split-string preamble (regexp-quote header-split))))
           (setq preamble (car preamble-parts)