[Fix #2424] Fallback to lein on jack-in when clojure is missing

The Clojure CLI tools are not available on Windows yet, so for them
`clojure-cli` is a poor default. Now we set the default in a dynamic
fashion.

Author: bbatsov

CHANGELOG.md

@@ -32,6 +32,7 @@
 * [#2307](https://github.com/clojure-emacs/cider/pull/2307): Use a better error when a cljs repl form cannot be found.
 * Fix the broken test selector functionality.
 * [#2291](https://github.com/clojure-emacs/cider/issues/2291): `cider-use-tooltips` custom variable works as expected.
+* [#2424](https://github.com/clojure-emacs/cider/issues/2424): Fallback to `lein` as the default jack-in command when `clojure` is not present.
 
 ### Changes
 

cider.el

@@ -214,12 +214,14 @@ By default we favor the project-specific shadow-cljs over the system-wide."
   :safe #'stringp
   :package-version '(cider . "0.10.0"))
 
-(defcustom cider-jack-in-default "clojure-cli"
+(defcustom cider-jack-in-default (if (executable-find "clojure") "clojure-cli" "lein")
   "The default tool to use when doing `cider-jack-in' outside a project.
 This value will only be consulted when no identifying file types, i.e.
 project.clj for leiningen or build.boot for boot, could be found.
 
-As the Clojure CLI is bundled with Clojure itself, it's the default."
+As the Clojure CLI is bundled with Clojure itself, it's the default.
+In the absence of the Clojure CLI (e.g. on Windows), we fallback
+to Leiningen."
   :type '(choice (const "lein")
                  (const "boot")
                  (const "clojure-cli")
@@ -1369,6 +1371,8 @@ PROJECT-DIR defaults to the current project."
                             choices nil t nil nil default))
           (choices
            (car choices))
+          ;; if we're outside a project we fallback to whatever tool
+          ;; is specified in `cider-jack-in-default' (normally clojure-cli)
           (t cider-jack-in-default))))