nls-global.mk: search build dir for source files, too

In VPATH builds, the build directory was not being searched for files in
GETTEXT_FILES, leading to failure to construct the .pot files.  This has
bit me all along, but never hard enough to get it fixed; I suppose not a
lot of people uses VPATH and NLS-enabled builds, and those that do,
don't do "make update-po" often.

This is a longstanding problem, so backpatch all the way back.

src/nls-global.mk

@@ -76,7 +76,7 @@ all-po: $(MO_FILES)
 ifeq ($(word 1,$(GETTEXT_FILES)),+)
 po/$(CATALOG_NAME).pot: $(word 2, $(GETTEXT_FILES)) $(MAKEFILE_LIST)
 ifdef XGETTEXT
-	$(XGETTEXT) -D $(srcdir) -n $(addprefix -k, $(GETTEXT_TRIGGERS)) $(addprefix --flag=, $(GETTEXT_FLAGS)) -f $<
+	$(XGETTEXT) -D $(srcdir) -D . -n $(addprefix -k, $(GETTEXT_TRIGGERS)) $(addprefix --flag=, $(GETTEXT_FLAGS)) -f $<
 else
 	@echo "You don't have 'xgettext'."; exit 1
 endif
@@ -85,7 +85,7 @@ po/$(CATALOG_NAME).pot: $(GETTEXT_FILES) $(MAKEFILE_LIST)
 # Change to srcdir explicitly, don't rely on $^.  That way we get
 # consistent #: file references in the po files.
 ifdef XGETTEXT
-	$(XGETTEXT) -D $(srcdir) -n $(addprefix -k, $(GETTEXT_TRIGGERS)) $(addprefix --flag=, $(GETTEXT_FLAGS)) $(GETTEXT_FILES)
+	$(XGETTEXT) -D $(srcdir) -D . -n $(addprefix -k, $(GETTEXT_TRIGGERS)) $(addprefix --flag=, $(GETTEXT_FLAGS)) $(GETTEXT_FILES)
 else
 	@echo "You don't have 'xgettext'."; exit 1
 endif