[gnome-builder] meson-templates: fix gtk-sharp3 template

[Christian Hergert <chergert src gnome org>]

commit 7ece0e2eb85aaf3cf0e5ca913f78e7edb80c33b8
Author: Christian Hergert <chergert redhat com>
Date:   Tue Oct 30 16:16:11 2018 -0700

    meson-templates: fix gtk-sharp3 template
    
    Some changes occurred recently, which make supporting this rather pesky.
    However, this gets stuff building again. We have to rely on 3.28 instead
    of 3.30 because of the freedesktop SDK extension. That has too old of
    meson, so we need to install our own meson of 0.48.1.
    
    Fixes #683

 .../meson-templates/resources/flatpak-gtksharp.json.tmpl | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/src/plugins/meson-templates/resources/flatpak-gtksharp.json.tmpl 
b/src/plugins/meson-templates/resources/flatpak-gtksharp.json.tmpl
index 43d84537b..4c7788932 100644
--- a/src/plugins/meson-templates/resources/flatpak-gtksharp.json.tmpl
+++ b/src/plugins/meson-templates/resources/flatpak-gtksharp.json.tmpl
@@ -33,10 +33,26 @@
                 "strip": true
             }
         },
+        {
+            "name" : "meson",
+            "buildsystem" : "simple",
+            "build-commands": ["pip3 install -I --prefix=/app ."],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/mesonbuild/meson.git";,
+                    "branch" : "0.48.1"
+                }
+            ]
+        },
         {
             "name": "gtk-sharp3",
             "buildsystem": "meson",
             "builddir": true,
+            "config-opts" : [
+                "--libdir=lib",
+                "-Dinstall=true"
+            ],
             "sources": [
                 {
                     "type": "git",