build: Move Hydra recipe to build-aux/hydra.

* hydra.scm: Rename to...
* build-aux/hydra/gnu-system.scm: ... this.
* Makefile.am (EXTRA_DIST): Adjust accordingly.

Makefile.am

@@ -127,7 +127,7 @@ EXTRA_DIST =					\
   ROADMAP					\
   TODO						\
   .dir-locals.el				\
-  hydra.scm					\
+  build-aux/hydra/gnu-system.scm		\
   build-aux/download.scm			\
   build-aux/sync-synopses.scm			\
   srfi/srfi-64.scm				\

build-aux/hydra/gnu-system.scm

@@ -31,7 +31,7 @@
 
   (and=> (assoc-ref (current-source-location) 'filename)
          (lambda (file)
-           (let ((dir (dirname file)))
+           (let ((dir (string-append (dirname file) "/../..")))
              (format (current-error-port) "prepending ~s to the load path~%"
                      dir)
              (set! %load-path (cons dir %load-path))))))