Set the path to virtualenvs in the Makefile

Also, version the virtualenv

src/pg/sql/01_version.sql

@@ -0,0 +1,12 @@
+-- Version number of the extension release
+CREATE OR REPLACE FUNCTION cdb_crankshaft_version()
+RETURNS text AS $$
+  SELECT '@@VERSION@@'::text;
+$$ language 'sql' IMMUTABLE STRICT;
+
+-- Internal identifier of the installed extension instence
+-- e.g. 'dev' for current development version
+CREATE OR REPLACE FUNCTION cdb_crankshaft_internal_version()
+RETURNS text AS $$
+  SELECT installed_version FROM pg_available_extensions where name='crankshaft' and pg_available_extensions IS NOT NULL;
+$$ language 'sql' IMMUTABLE STRICT;