ox-beamer: Support #+LATEX_COMPILER

* ox-beamer.el (org-beamer-template): Insert compiler and remove
  unused packages.

lisp/ox-beamer.el

@@ -827,31 +827,11 @@ holding export options."
      ;; Time-stamp.
      (and (plist-get info :time-stamp-file)
 	  (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
-     ;; 2. Document class and packages.
-     (let* ((class (plist-get info :latex-class))
-	    (class-options (plist-get info :latex-class-options))
-	    (header (nth 1 (assoc class org-latex-classes)))
-	    (document-class-string
-	     (and (stringp header)
-		  (if (not class-options) header
-		    (replace-regexp-in-string
-		     "^[ \t]*\\\\documentclass\\(\\(\\[[^]]*\\]\\)?\\)"
-		     class-options header t nil 1)))))
-       (if (not document-class-string)
-	   (user-error "Unknown LaTeX class `%s'" class)
-	 (org-latex-guess-babel-language
-	  (org-latex-guess-inputenc
-	   (org-element-normalize-string
-	    (org-splice-latex-header
-	     document-class-string
-	     org-latex-default-packages-alist
-	     org-latex-packages-alist nil
-	     (concat (org-element-normalize-string
-		      (plist-get info :latex-header))
-		     (org-element-normalize-string
-		      (plist-get info :latex-header-extra))))))
-	  info)))
      ;; LaTeX compiler
+     (org-latex--insert-compiler info)
+     ;; Document class and packages.
+     (org-latex--make-preamble info)
+     ;; Insert themes.
      (let ((format-theme
 	    (function
 	     (lambda (prop command)