Allow :manifest entries to contain functions.

sample.project.clj

@@ -181,7 +181,13 @@
                                 :bytes (:out (clojure.java.shell/sh
                                               "git" "log" "-n" "1"))})}]
   ;; Set arbitrary key/value pairs for the jar's manifest.
-  :manifest {"Project-awesome-level" "super-great"}
+  :manifest {"Project-awesome-level" "super-great"
+             ;; function values will be called with the project as an argument.
+             "Class-Path" ~#(clojure.string/join
+                             java.io.File/pathSeparatorChar
+                             (leiningen.core.classpath/get-classpath %))
+             ;; symbol values will be resolved to find a function to call.
+             "Grunge-level" my.plugin/calculate-grunginess}
   ;; You can set JVM-level options here.
   :jvm-opts ["-Xmx1g"]
   ;; Control the context in which your project code is evaluated.

src/leiningen/jar.clj

@@ -84,12 +84,16 @@
    "Built-By" (System/getProperty "user.name")
    "Build-Jdk" (System/getProperty "java.version")})
 
+(defn- manifest-entry [project manifest [k v]]
+  (cond (symbol? v) (manifest-entry project manifest [k (resolve v)])
+        (fn? v) (manifest-entry project manifest [k (v project)])
+        :else (str manifest "\n" (name k) ": " v)))
+
 (defn ^:internal make-manifest [project]
   (Manifest.
    (ByteArrayInputStream.
     (.getBytes
-     (reduce (fn [manifest [k v]]
-               (str manifest "\n" k ": " v))
+     (reduce (partial manifest-entry project)
              "Manifest-Version: 1.0"
              (merge default-manifest (:manifest project)
                     ;; (when (:shell-wrapper project)