libxml2: make the python dependency optional

Also turn it off by default since it is usually not necessary. Anything
needing the Python bindings should declare it explicitly.

var/spack/packages/libxml2/package.py

@@ -9,13 +9,20 @@ class Libxml2(Package):
 
     version('2.9.2', '9e6a9aca9d155737868b3dc5fd82f788')
 
-    extends('python')
+    variant('python', default=False, description='Enable Python support')
+
+    extends('python', when='+python')
     depends_on('zlib')
     depends_on('xz')
 
     def install(self, spec, prefix):
+        if '+python' in spec:
+            python_arg = "--with-python=%s" % spec['python'].prefix
+        else:
+            python_arg = "--without-python"
+
         configure("--prefix=%s" % prefix,
-                  "--with-python=%s" % spec['python'].prefix)
+                  python_arg)
 
         make()
         make("install")