Add setup_extension_environment() method.

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

lib/spack/spack/package.py

@@ -783,6 +783,12 @@ def do_install(self, **kwargs):
                 self.stage.chdir_to_source()
                 build_env.setup_package(self)
 
+                # Allow extendees to further set up the environment.
+                for ext_name in self.extendees:
+                    ext_spec = self.spec[ext_name]
+                    ext_spec.package.setup_extension_environment(
+                        self.module, ext_spec, self.spec)
+
                 if fake_install:
                     self.do_fake_install()
                 else:
@@ -854,6 +860,30 @@ def module(self):
                           fromlist=[self.__class__.__name__])
 
 
+    def setup_extension_environment(self, module, spec, ext_spec):
+        """Called before the install() method of extensions.
+
+        Default implementation does nothing, but this can be
+        overridden by an extendable package to set up the install
+        environment for its extensions.  This is useful if there are
+        some common steps to installing all extensions for a
+        certain package.
+
+        Some examples:
+
+        1. Installing python modules generally requires PYTHONPATH to
+           point to the lib/pythonX.Y/site-packages directory in the
+           module's install prefix.  This could set that variable.
+
+        2. Extensions often need to invoke the 'python' interpreter
+           from the Python installation being extended.  This routine can
+           put a 'python' Execuable object in the module scope for the
+           extension package to simplify extension installs.
+
+        """
+        pass
+
+
     def install(self, spec, prefix):
         """Package implementations override this with their own build configuration."""
         raise InstallError("Package %s provides no install method!" % self.name)