Make the Makefile like other contrib modules.

This makes it easier to unpack a temporal release into the `contrib` directory
of an unpacked PostgreSQL source tarball and to install it like any other
contrib module.

Makefile

@@ -3,7 +3,13 @@ MODULES = period
 DATA_built = period.sql
 DATA = uninstall_period.sql
 
+ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
-
+else
+subdir = contrib/temporal
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif