[gnome-sdk-images] Install meson in the right prefix

[Mathieu Bridon <mbridon src gnome org>]

commit b76dbade1c42080059121357056f7d003c717663
Author: Mathieu Bridon <bochecha daitauha fr>
Date:   Tue Aug 1 13:00:59 2017 +0200

    Install meson in the right prefix
    
    Silly me, this is an Sdk, not an app.

 org.gnome.Sdk.json.in |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 5a7696e..3d4af53 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -72,7 +72,10 @@
             "name": "meson",
             "buildsystem": "simple",
             "build-commands": [
-                "python3 setup.py install --prefix=/app"
+                "python3 setup.py install"
+            ],
+            "ensure-writable": [
+                "/lib/python3.5/site-packages/easy-install.pth"
             ],
             "cleanup-platform": ["*"],
             "sources": [