Bug#940717: gtk-d FTBFS on armhf, tries to build with ldc but it's not installed.

[peter green]

found 940717 3.8.5-1
tags 940717 +bullseye
tags 940717 +sid

Since the version of default-d-compiler which uses gdc on armhf migrated to 
testing, gtk-d also fails to build on armhf in testing.