Add cross-compile script

That will simply generate the win64 artifacts for those of us that run
DF under Proton on Steam for Linux.

build/.gitignore

@@ -6,3 +6,4 @@ DF_PATH.txt
 _CPack_Packages
 *.tar.*
 .cmake
+win64-cross

build/build-win64-from-linux.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env sh
+
+# Number of jobs == core count
+jobs=$(grep -c ^processor /proc/cpuinfo)
+
+# Calculate absolute paths for docker to do mounts
+srcdir=$(realpath "$(dirname "$(readlink -f "$0")")"/..)
+
+cd "$srcdir"/build
+
+mkdir -p win64-cross
+mkdir -p win64-cross/output
+
+# Assumes you built a container image called dfhack-build-msvc from
+# https://github.com/BenLubar/build-env/tree/master/msvc, see
+# docs/dev/compile/Compile.rst.
+#
+# NOTE: win64-cross is mounted in /src/build due to the hardcoded `cmake ..` in
+# the Dockerfile
+#
+# TODO: make this work for rootless docker, i.e. remove the sudo for those that
+# don't normally need to use sudo to run docker.
+if ! sudo docker run --rm -it -v "$srcdir":/src -v "$srcdir/build/win64-cross/":/src/build  \
+    --user buildmaster \
+    --name dfhack-win \
+    dfhack-build-msvc bash -c "cd /src/build && dfhack-configure windows 64 Release -DCMAKE_INSTALL_PREFIX=/src/build/output && dfhack-make -j$jobs install" \
+    ; then
+    echo
+    echo "Build failed"
+    exit 1
+else
+    echo
+    echo "Windows artifacts are at win64-cross/output. Copy or symlink them to"
+    echo "your steam DF directory to install dfhack (and optionally delete the"
+    echo "hack/ directory already present)"
+    echo
+    echo "Typically this can be done like this:"
+    echo "  cp -r win64-cross/output/* \"$HOME/.local/share/Steam/steamapps/common/Dwarf Fortress\""
+fi

docs/dev/compile/Compile.rst

@@ -284,8 +284,8 @@ addition to the normal ``CC`` and ``CXX`` flags above::
 
   export PATH=/usr/local/bin:$PATH
 
-Windows cross compiling from Linux
-==================================
+Windows cross compiling from Linux (running DF inside docker)
+=============================================================
 
 .. highlight:: bash
 
@@ -368,6 +368,62 @@ host when you want to reattach::
 If you edit code and need to rebuild, run ``dfhack-make`` and then ``ninja install``.
 That will handle all the wineserver management for you.
 
+Cross-compiling windows files for running DF in Steam for Linux
+===============================================================
+
+.. highlight:: bash
+
+If you wish, you can use Docker to build just the Windows files to copy to your
+existing Steam installation on Linux.
+
+.. contents::
+  :local:
+  :depth: 1
+
+Step 1: Build the MSVC builder image
+------------------------------------
+
+It'll be called ``dfhack-build-msvc:latest`` after it's done building::
+
+   git clone https://github.com/BenLubar/build-env.git
+   cd build-env/msvc
+   docker build --build-arg BUILDER_UID=$(id -u) -t dfhack-build-msvc .
+
+The ``BUILDER_UID`` argument is used to make sure the build user can access your
+source code directory inside the container.
+
+The docker build takes a while, but only needs to be done once, unless the build
+environment changes.
+
+Step 2: Get dfhack, and run the build script
+--------------------------------------------
+
+Check out ``dfhack`` into another directory, and run the build script::
+
+   git clone https://github.com/DFHack/dfhack.git
+   cd dfhack
+   git submodule update --init --recursive
+   cd build
+   ./build-win64-from-linux.sh
+
+The last step may ask you for your ``sudo`` password -- if you are able to run
+``docker`` without ``sudo`` normally, you can remove the ``sudo`` from the build
+script.
+
+The script will mount your host's ``dfhack`` directory to docker, use it to
+build the artifacts in ``build/win64-cross``, and put all the files needed to
+install in ``build/win64-cross/output``.
+
+Step 3: install dfhack to your Steam DF install
+-----------------------------------------------
+As the script will tell you, you can then copy the files into your DF folder::
+
+   # Optional -- remove the old hack directory in case we leave files behind
+   rm ~/.local/share/Steam/steamapps/common/"Dwarf Fortress"/hack
+   cp -r win64-cross/output/* ~/.local/share/Steam/steamapps/common/"Dwarf Fortress"/
+
+Afterward, just run DF as normal.
+
 .. _note-offline-builds:
 
 Building DFHack Offline