Add setup_extension_environment() method.

- lets packages do some setup before their extensions run install()

var/spack/packages/py-setuptools/package.py

@@ -10,10 +10,4 @@ class PySetuptools(Package):
     extends('python')
 
     def install(self, spec, prefix):
-        site_packages_dir = "%s/lib/python2.7/site-packages" % prefix
-        mkdirp(site_packages_dir)
-
-        env['PYTHONPATH'] = site_packages_dir
-
-        python = which('python')
         python('setup.py', 'install', '--prefix=%s' % prefix)

var/spack/packages/python/package.py

@@ -1,5 +1,5 @@
 from spack import *
-
+import os
 
 class Python(Package):
     """The Python programming language."""
@@ -26,3 +26,24 @@ def install(self, spec, prefix):
                   "--enable-shared")
         make()
         make("install")
+
+
+    def setup_extension_environment(self, module, spec, ext_spec):
+        """Called before python modules' install() methods.
+
+        In most cases, extensions will only need to have one line::
+
+            python('setup.py', 'install', '--prefix=%s' % prefix)
+        """
+        # Python extension builds can have a global python executable function
+        module.python = Executable(join_path(spec.prefix.bin, 'python'))
+
+        # Add variables for lib/pythonX.Y and lib/pythonX.Y/site-packages dirs.
+        module.python_lib_dir = join_path(ext_spec.prefix.lib, 'python%d.%d' % self.version[:2])
+        module.site_packages_dir = join_path(module.python_lib_dir, 'site-packages')
+
+        # Add site packages directory to the PYTHONPATH
+        os.environ['PYTHONPATH'] = module.site_packages_dir
+
+        # Make the site packages directory if it does not exist already.
+        mkdirp(module.site_packages_dir)