Fix update-po for the PGXS case

The original formulation failed to take into account the fact that for
the PGXS case, the source dir is not $(top_srcdir), so it ended up not
doing anything.  Handle it explicitly.

Author: Ryo Matsumura <matsumura.ryo@fujitsu.com>
Reviewed-by: Bryan Green <dbryan.green@gmail.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/TYCPR01MB113164770FB0B0BE6ED21E68EE8DCA@TYCPR01MB11316.jpnprd01.prod.outlook.com

src/nls-global.mk

@@ -142,8 +142,13 @@ init-po: po/$(CATALOG_NAME).pot
 # For performance reasons, only calculate these when the user actually
 # requested update-po or a specific file.
 ifneq (,$(filter update-po %.po.new,$(MAKECMDGOALS)))
+ifdef PGXS
+ALL_LANGUAGES := $(shell find . -name '*.po' -print | sed 's,^.*/\([^/]*\).po$$,\1,' | LC_ALL=C sort -u)
+all_compendia := $(shell find . -name '*.po' -print | LC_ALL=C sort)
+else
 ALL_LANGUAGES := $(shell find $(top_srcdir) -name '*.po' -print | sed 's,^.*/\([^/]*\).po$$,\1,' | LC_ALL=C sort -u)
 all_compendia := $(shell find $(top_srcdir) -name '*.po' -print | LC_ALL=C sort)
+endif
 else
 ALL_LANGUAGES = $(AVAIL_LANGUAGES)
 all_compendia = FORCE