Set the path to virtualenvs in the Makefile

Also, version the virtualenv

src/pg/sql/02_py.sql

@@ -0,0 +1,23 @@
+CREATE OR REPLACE FUNCTION _cdb_crankshaft_virtualenvs_path()
+RETURNS text
+AS $$
+  BEGIN
+    -- RETURN '/opt/virtualenvs/crankshaft';
+    RETURN '@@VIRTUALENV_PATH@@';
+  END;
+$$ language plpgsql IMMUTABLE STRICT;
+
+-- Use the crankshaft python module
+CREATE OR REPLACE FUNCTION _cdb_crankshaft_activate_py()
+RETURNS VOID
+AS $$
+    import os
+    # plpy.notice('%',str(os.environ))
+    # activate virtualenv
+    crankshaft_version = plpy.execute('SELECT cdb_crankshaft.cdb_crankshaft_internal_version()')[0]['cdb_crankshaft_internal_version']
+    base_path = plpy.execute('SELECT cdb_crankshaft._cdb_crankshaft_virtualenvs_path()')[0]['_cdb_crankshaft_virtualenvs_path']
+    default_venv_path = os.path.join(base_path, crankshaft_version)
+    venv_path =  os.environ.get('CRANKSHAFT_VENV', default_venv_path)
+    activate_path = venv_path + '/bin/activate_this.py'
+    exec(open(activate_path).read(), dict(__file__=activate_path))
+$$ LANGUAGE plpythonu;