Fix #244: errors on uninstall

- Extension logic didn't take conditional deps into account.

- Extension methods now check for whether the extnesion is
  in the extendee map AND whether the dependency is actually present
  in the spec yet.

lib/spack/spack/hooks/extensions.py

@@ -27,9 +27,7 @@
 
 
 def pre_uninstall(pkg):
-    # Need to do this b/c uninstall does not automatically do it.
+    assert(pkg.spec.concrete)
-    # TODO: store full graph info in stored .spec file.
-    pkg.spec.normalize()
 
     if pkg.is_extension:
         if pkg.activated:

lib/spack/spack/package.py

@@ -487,7 +487,13 @@ def extendee_spec(self):
             if name == dep.name:
                 return dep
 
-        # Otherwise return the spec from the extends() directive
+        # if the spec is concrete already, then it extends something
+        # that is an *optional* dependency, and the dep isn't there.
+        if self.spec._concrete:
+            return None
+        else:
+            # If it's not concrete, then return the spec from the
+            # extends() directive since that is all we know so far.
             spec, kwargs = self.extendees[name]
             return spec
 
@@ -497,18 +503,28 @@ def extendee_args(self):
         """Spec of the extendee of this package, or None if it is not an extension."""
         if not self.extendees:
             return None
+
+        # TODO: allow multiple extendees.
         name = next(iter(self.extendees))
         return self.extendees[name][1]
 
 
     @property
     def is_extension(self):
-        return len(self.extendees) > 0
+        # if it is concrete, it's only an extension if it actually
+        # dependes on the extendee.
+        if self.spec._concrete:
+            return self.extendee_spec is not None
+        else:
+            # If not, then it's an extension if it *could* be an extension
+            return bool(self.extendees)
 
 
     def extends(self, spec):
-        return (spec.name in self.extendees and
+        if not spec.name in self.extendees:
-                spec.satisfies(self.extendees[spec.name][0]))
+            return False
+        s = self.extendee_spec
+        return s and s.satisfies(spec)
 
 
     @property