I have probably heavily over-engineered this; it helps me derive the default path for "share/" and "lisp/" directories.
I couldn't find any internal variable that holds those values.
Here's the code:
(let* ((bin-dir (when (and invocation-directory
(file-exists-p invocation-directory))
(file-truename invocation-directory)))
(prefix-dir (when bin-dir
(replace-regexp-in-string "bin/\\'" "" bin-dir)))
(share-dir (when prefix-dir
(concat prefix-dir "share/")))
(lisp-dir-1 (when share-dir ;Possibility where the lisp dir is something like ../emacs/26.0.50/lisp/
(concat share-dir "emacs/"
;; If `emacs-version' is x.y.z.w, remove the ".w" portion
;; Though, this is not needed and also will do nothing in emacs 26+
;;
http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=22b2207471807bda86534b4faf1a29b3a6447536 (replace-regexp-in-string "\\([0-9]+\\.[0-9]+\\.[0-9]+\\).*" "\\1" emacs-version)
"/lisp/")))
(lisp-dir-2 (when share-dir ;Possibility where the lisp dir is something like ../emacs/25.2/lisp/
(concat share-dir "emacs/"
(replace-regexp-in-string "\\([0-9]+\\.[0-9]+\\).*" "\\1" emacs-version)
"/lisp/"))))
(defvar modi/default-share-directory (when (file-exists-p share-dir)
share-dir)
"Share directory for this Emacs installation.")
(defvar modi/default-lisp-directory (cond
((file-exists-p lisp-dir-1)
lisp-dir-1)
((file-exists-p lisp-dir-2)
lisp-dir-2)
(t
nil))
"Directory containing lisp files for the Emacs installation.
This value must match the path to the lisp/ directory of the
Emacs installation. If Emacs is installed using
--prefix=\"${PREFIX_DIR}\" this value would typically be
\"${PREFIX_DIR}/share/emacs/<VERSION>/lisp/\"."))