[gnome-builder] manifest: Enable flatpak_build option for devhelp

[Christian Hergert <chergert src gnome org>]

commit b546f6d7f7df44bc1fa3a32ba1d9d1adfa5f67f6
Author: vanadiae <vanadiae35 gmail com>
Date:   Mon Jul 5 19:54:02 2021 +0200

    manifest: Enable flatpak_build option for devhelp
    
    It should automatically add the /run/host paths corresponding to the host's
    documentations. Builder already has access to them with the --filesystem=host
    permission.

 build-aux/flatpak/org.gnome.Builder.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 7775252c0..7f0fbb7ad 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -310,7 +310,8 @@
             "builddir" : true,
             "config-opts" : [
                 "--libdir=/app/lib",
-                "--buildtype=debugoptimized"
+                "--buildtype=debugoptimized",
+                "-Dflatpak_build=true"
             ],
             "cleanup" : [
                 "/bin/*",