[gnome-builder] build: add meson 0.50

[Christian Hergert <chergert src gnome org>]

commit 46acc0c90a273e1c8028d503cd375675bf7437c9
Author: Christian Hergert <chergert redhat com>
Date:   Mon Mar 25 17:05:07 2019 -0700

    build: add meson 0.50
    
    Until the runtime/SDK supports meson 0.50, we can bundle it
    ourselves for 3.33 development. We want to do this early so that
    we can clean up the build system in time for distribution
    packagers to get ahead of things.

 build-aux/flatpak/org.gnome.Builder.json | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 86a63ff10..363c926b8 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -548,6 +548,20 @@
                 }
             ]
         },
+        {
+            "name" : "meson",
+            "buildsystem" : "simple",
+            "build-commands": [
+                "pip3 install --no-index --find-links=\"file://${PWD}\" --prefix=/app -I meson"
+            ],
+            "sources" : [
+                {
+                    "type" : "file",
+                    "url" : 
"https://files.pythonhosted.org/packages/3f/03/8785cb9fe3c499e5b3fedbb51dae0a608ea3dd0a8fc81cd0ae2237ab054a/meson-0.50.0.tar.gz";,
+                    "sha256" : "13e034b8e74fd39cb4a833915343bcb291b0337374d0537b76922605dad28648"
+                }
+            ]
+        },
         {
             "name" : "gnome-builder",
             "buildsystem" : "meson",