D compiler package is incomplete

[Bruno Haible]

On Cygwin 3.6.3, I have the packages
  gcc-core
  gcc-g++
  gcc-gdc
installed, all in version 11.5.0-1.

But the D compiler is dysfunctional: The attempt to compile any D source code
fails. E.g.:

$ echo > empty.d
$ gdc -c empty.c
d21: error: cannot find source code for runtime library file 'object.d'
d21: note: dmd might not be correctly installed. Run 'dmd -man' for installation instructions.
d21: note: config file: not found
$ echo $?
1

When I build GCC myself from source for other platforms, the D runtime
library is called 'libgphobos'. Without this library, both 'gdc' and 'd21'
are useless.

Bruno