Add a setup hook that automatically sets up $CLASSPATH

All JARs in $pkg/share/java (for each $pkg in the build inputs) are
added to $CLASSPATH.  Thus, you can say

  buildInputs = [ setJavaClassPath someJavaDependency ];

and the JARs in someJavaDependency will be found automatically by
tools like javac or ant.

Note that the manual used to say that JARs should be installed in
lib/java; this is now share/java, following the Debian policy:

  http://www.debian.org/doc/packaging-manuals/java-policy/x110.html

The directory share/java makes more sense because JARs are
architecture-independent.  (Also, a quick grep shows that we were not
exactly consistent about this in Nixpkgs.)

pkgs/build-support/setup-hooks/set-java-classpath.sh

@@ -0,0 +1,13 @@
+# This setup hook adds every JAR in the share/java subdirectories of
+# the build inputs to $CLASSPATH.
+
+export CLASSPATH
+
+addPkgToClassPath () {
+    local jar
+    for jar in $1/share/java/*.jar; do
+        export CLASSPATH=''${CLASSPATH}''${CLASSPATH:+:}''${jar}
+    done
+}
+
+envHooks=(''${envHooks[@]} addPkgToClassPath)