Refuse to create new extension if legacy code is present on database

Closes #20

Makefile

@@ -25,7 +25,7 @@ PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
 
 $(EXTENSION)--$(EXTVERSION).sql: $(CDBSCRIPTS) cartodb_hooks.sql cartodb_version.sql Makefile 
-	echo '\echo Use "CREATE EXTENSION cartodb" to load this file. \quit' > $@
+	echo '\echo Use "CREATE EXTENSION $(EXTENSION)" to load this file. \quit' > $@
 	cat $(CDBSCRIPTS) | \
     sed -e 's/\<public\./cartodb./g' \
         -e 's/:DATABASE_USERNAME/cdb_org_admin/g' >> $@

cartodb_version.sql.in

@@ -1,3 +1,5 @@
+DO $$ BEGIN IF EXISTS (SELECT * FROM pg_proc p, pg_namespace n WHERE p.proname = 'cdb_transformtowebmercator' AND p.pronamespace = n.oid AND n.nspname = 'public') THEN RAISE EXCEPTION 'Use CREATE EXTENSION cartodb FROM unpackaged'; END IF; END; $$ LANGUAGE 'plpgsql'; -- forbid duplicated extension
+
 CREATE OR REPLACE FUNCTION cartodb.CDB_version()
 RETURNS text AS $$
   SELECT '@@VERSION@@'::text;

util/create_from_unpackaged.sh

@@ -4,7 +4,7 @@ ver=$1
 input=cartodb--${ver}.sql
 output=cartodb--unpackaged--${ver}.sql
 
-cat ${input} > ${output}
+cat ${input} | grep -v 'duplicated extension$' > ${output}
 
 # Migrate CDB_TableMetadata
 cat >> ${output} <<'EOF'