[gimp] po: explain why `make update-po` will fail on a VPATH build.

[Jehan Pagès <jehanp src gnome org>]

commit c4bb7089d1b81c26b3890894093828b021f80263
Author: Jehan <jehan girinstud io>
Date:   Sun Oct 2 01:53:29 2016 +0200

    po: explain why `make update-po` will fail on a VPATH build.
    
    Commit c96006919 worked around a still-existing bug (well at least the
    bug report is still open) of intltool with automake 1.15 when the build
    and source directories are different (VPATH build) by replacing
    `intltool-update` by `true`. But because of this `make update-po` fails
    on such a build and the make output is totally unclear at why. Add an
    error message when this happens.

 po/Makefile.in.in |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/po/Makefile.in.in b/po/Makefile.in.in
index b2d98c9..9add436 100644
--- a/po/Makefile.in.in
+++ b/po/Makefile.in.in
@@ -97,6 +97,11 @@ all-yes: $(CATALOGS)
 all-no:
 
 $(GETTEXT_PACKAGE).pot: $(POTFILES)
+       @if test "x$(INTLTOOL_UPDATE)" = "x/bin/true"; then \
+               echo "Error: po generation deactivated on VPATH builds."; \
+               echo "       See commit c96006919."; \
+               false; \
+       fi
        $(GENPOT)
 
 install: install-data