[gimp] build: fix Webkit build in Flatpak package.

[Jehan Pagès <jehanp src gnome org>]

commit afc69f83ae160000f73911af056d60b733aa6a63
Author: Jehan <jehan girinstud io>
Date:   Thu Feb 9 22:19:33 2017 +0100

    build: fix Webkit build in Flatpak package.
    
    For some reason, Webkit would not build with Python 2 on a different
    machine though it did on mine. Let's just force Python 3 usage (for
    build only, not as a runtime dependency) by setting the $PYTHON
    environment variable during this build. Anyway Python 3 is available
    from the base build so it's not a new dependency.

 build/flatpak/org.gimp.GIMP.json |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/build/flatpak/org.gimp.GIMP.json b/build/flatpak/org.gimp.GIMP.json
index 948734b..28b3e45 100644
--- a/build/flatpak/org.gimp.GIMP.json
+++ b/build/flatpak/org.gimp.GIMP.json
@@ -265,6 +265,11 @@
             "name": "webkitgtk",
             "config-opts": [ "--with-gtk=2.0", "--disable-webkit2", "--disable-geolocation", 
"--disable-video",
                              "--disable-spellcheck", "--disable-credential-storage", "--disable-web-audio" ],
+            "build-options" : {
+                "env": {
+                    "PYTHON": "/usr/bin/python3"
+                }
+            },
             "cleanup": [ "/share/doc", "/bin" ],
             "sources": [
                 {