This should help get the runtime built.
FOR D1:
=======
1) Do a checkout of tango trunk into the ldc root dir (along dmd, gen, runtime etc).

    *   $ ls
    *   dmd dmd2 gen ir ...
    *   $ svn co http://svn.dsource.org/projects/tango/trunk tango

2) Patch the runtime

    *   cd runtime
    *   sh patch-tango.sh

3) Compile the runtime

    *   sh build.sh

FOR D2:
=======
Highly experimental!!!

1) Do a checkout of druntime trunk into the ldc root dir (along dmd, gen, runtime etc).

    *   $ ls
    *   dmd dmd2 gen ir ...
    *   $ svn co http://svn.dsource.org/projects/druntime/trunk druntime

2) Patch druntime

    *   cd runtime
    *   sh patch-druntime.sh

3) Compile druntime

    *   cd ../druntime/src
    *   sh build-ldc.sh

=======
Enjoy!