Conditionally respect sclang-vars

Only when compiling with cmake will sclang-vars.el be written. This
commit moves relevant constants into customizable vars. Only if the
`sclang-system-data-dir` exists is it respected, otherwise it will guess
the proper location by looking at the system-type.

The purpose of this is to make it possible to build and distribute a
package independent of the SuperCollider build process, while ensuring
that those users who have become accustomed to the current install
process are not disrupted.

.gitignore

@@ -0,0 +1 @@
+.DS_Store

el/sclang-help.el

@@ -24,15 +24,29 @@
 (require 'sclang-interp)
 (require 'sclang-language)
 (require 'sclang-mode)
-(require 'sclang-vars)
+(require 'sclang-vars nil 'ignore-missing-file)
 (require 'sclang-minor-mode)
 
-(defcustom sclang-help-directory "~/SuperCollider/Help"
+(defun sclang-system-root ()
-  "*Directory where the SuperCollider help files are kept. OBSOLETE."
+  "Find the common install location for the platform."
+  (cond
+   ((boundp 'sclang-system-data-dir)
+    sclang-system-data-dir)
+
+   ((eql 'darwin system-type)
+    (expand-file-name "~/Library/Application Support/SuperCollider"))
+
+   ((and (eql 'gnu/linux system-type)
+         (file-exists-p "/usr/local/share/SuperCollider"))
+    "/usr/local/share/SuperCollider")
+
+   ((eql 'gnu/linux system-type)
+    "/usr/share/SuperCollider")))
+
+(defcustom sclang-system-help-dir (expand-file-name "Help" (sclang-system-root))
+  "Installation dependent help directory."
   :group 'sclang-interface
-  :version "21.3"
+  :type 'directory)
-  :type 'directory
-  :options '(:must-match))
 
 (defcustom sclang-help-path (list sclang-system-help-dir
 				  "~/.local/share/SuperCollider/Help")
@@ -41,6 +55,11 @@
   :version "21.4"
   :type '(repeat directory))
 
+(defcustom sclang-system-extension-dir (expand-file-name "Extensions" (sclang-system-root))
+  "Installation dependent extension directory."
+  :group 'sclang-interface
+  :type 'directory)
+
 (defconst sclang-extension-path (list sclang-system-extension-dir
 				      "~/.local/share/SuperCollider/Extensions")
   "List of SuperCollider extension directories.")

el/sclang-vars.el.in

@@ -16,19 +16,23 @@
 
 ;; You should have received a copy of the GNU General Public License
 ;; along with GNU Emacs; see the file COPYING.  If not, write to
-;; the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, 
+;; the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
 ;; Boston, MA 02110-1301, USA.
 
+;;; Commentary:
+;;
+;; This file is included in the distribution in order to convey system
+;; installation variables which are known at compile time, but only when
+;; the library is installed with cmake along with SuperCollider source.
+;;
+;; When using the stand-alone package, this file can be ignored and
+;; the constants it defines will have no effect.
+
 ;;; Code:
 
 (defconst sclang-system-data-dir "@PKG_DATA_DIR@"
-  "Installation dependent data directory.")
+  "Installation dependent data directory.
-
+Bound only when library is installed with SuperCollider.")
-(defconst sclang-system-help-dir "@PKG_DATA_DIR@/Help"
-  "Installation dependent help directory.")
-
-(defconst sclang-system-extension-dir "@PKG_DATA_DIR@/Extensions"
-  "Installation dependent extension directory.")
 
 (provide 'sclang-vars)
 ;;; sclang-vars.el ends here