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/libmng/package.py

@@ -0,0 +1,23 @@
+from spack import *
+
+class Libmng(Package):
+    """libmng -THE reference library for reading, displaying, writing
+       and examining Multiple-Image Network Graphics.  MNG is the animation
+       extension to the popular PNG image-format."""
+    homepage = "http://sourceforge.net/projects/libmng/"
+    url      = "http://downloads.sourceforge.net/project/libmng/libmng-devel/2.0.2/libmng-2.0.2.tar.gz"
+
+    version('2.0.2', '1ffefaed4aac98475ee6267422cbca55')
+
+    depends_on("jpeg")
+    depends_on("zlib")
+    depends_on("lcms")
+
+    def patch(self):
+        # jpeg requires stdio to beincluded before its headrs.
+        filter_file(r'^(\#include \<jpeglib\.h\>)', '#include<stdio.h>\n\\1', 'libmng_types.h')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")