Make D template work even if dub is not available.

mesonbuild/templates/dlangtemplates.py

@@ -88,13 +88,15 @@ test('{test_name}', test_exe)
 # Make this library usable from the Dlang
 # build system.
 dlang_mod = import('dlang')
-dlang_mod.generate_dub_file(meson.project_name().to_lower(), meson.source_root(),
+if find_program('dub', required: false).found()
+  dlang_mod.generate_dub_file(meson.project_name().to_lower(), meson.source_root(),
     name : meson.project_name(),
     license: meson.project_license(),
     sourceFiles : '{source_file}',
     description : 'Meson sample project.',
     version : '{version}',
-)
+  )
+endif
 '''
 
 def create_exe_d_sample(project_name, project_version):