My local hacks to make xls compile on nix.

In your xls checkout, link the shell.nix to your
directory (and the patch for convenience).

Apply the patches
for f in nix-*.patch ; do patch -p1 < $f ; done

Currently, this does not properly work with
the downloaded llvm toolchain, as that can't deal
with things found in /etc/os-release

TODO: create an /etc/os-release file as part of
the FHS.

For now, we just compile with gcc (which creates
some issues and can't compile all files).

# Build with --keep_going as not everything is
# compiling yet.
bazel build -c opt ... --keep_going

Ideally, we'd use something like shell.nix.ideal,
but the bad interaction of bazel and Nix fighting
for authority about what is hermetic makes this
require a bit more work.