Fix SPACK-40: Finish adding variant directive.

- Variants are now declarable in packages using the variant() directive.
- Variants are checked - you can't just ask for a random variant, it has to be declared.
- conditional logic (@when, if, '+debug' in spec, etc.) still required in package to
  implement variant.

var/spack/mock_packages/mpich/package.py

@@ -30,6 +30,9 @@ class Mpich(Package):
     list_url   = "http://www.mpich.org/static/downloads/"
     list_depth = 2
 
+    variant('debug', default=False,
+            description="Compile MPICH with debug flags.")
+
     version('3.0.4', '9c5d5d4fe1e17dd12153f40bc5b6dbc0')
     version('3.0.3', 'foobarbaz')
     version('3.0.2', 'foobarbaz')