New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin/packages/gtkplus/package.py

@@ -0,0 +1,22 @@
+from spack import *
+
+class Gtkplus(Package):
+    """The GTK+ 2 package contains libraries used for creating graphical user interfaces for applications."""
+    homepage = "http://www.gtk.org"
+
+    version('2.24.25', '612350704dd3aacb95355a4981930c6f',
+            url="http://ftp.gnome.org/pub/gnome/sources/gtk+/2.24/gtk+-2.24.25.tar.xz")
+
+    depends_on("atk")
+    depends_on("gdk-pixbuf")
+    depends_on("pango")
+
+    def patch(self):
+        # remove disable deprecated flag.
+        filter_file(r'CFLAGS="-DGDK_PIXBUF_DISABLE_DEPRECATED $CFLAGS"',
+                    '', 'configure', string=True)
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")