In order to build darktable for Windows there is a chance to cross compile it
on Linux. The result is currently not particularly usable, but still, if you
are curious you could give it a try.

The tests have been done cross compiling on openSUSE. Since it's using a
virtualized installation only used for this stuff you can safely do everything
as root. It's also enough to install a server setup without X.

- Grab the openSUSE install ISO from http://software.opensuse.org/122/en
  (I used the 64 Bit version).
- Install in Virtualbox.
- Prepare the system:

------------------ snip ------------------
#!/bin/sh

# add the cross compiling repositoryies
 # regular stuff
 zypper ar http://download.opensuse.org/repositories/windows:/mingw:/win64/openSUSE_12.2/windows:mingw:win64.repo
 # extra repository for exiv2 and lensfun
 zypper ar http://download.opensuse.org/repositories/home:/sergeyopensuse:/branches:/windows:/mingw:/win64/openSUSE_12.2/home:sergeyopensuse:branches:windows:mingw:win64.repo

# install the needed tools like cross compilers
zypper install bash-completion mingw64-cross-gcc mingw64-cross-gcc-c++ cmake binutils git libxslt

# install the cross compiled libraries
zypper install mingw64-win_iconv mingw64-win_iconv-devel mingw64-gtk2 \
               mingw64-gtk2-devel mingw64-libxml2 mingw64-libxml2-devel \
               mingw64-libgomp mingw64-lensfun mingw64-lensfun-devel \
               mingw64-pthreads mingw64-pthreads-devel mingw64-librsvg \
               mingw64-librsvg-devel mingw64-libsqlite mingw64-libsqlite-devel \
               mingw64-libexiv2 mingw64-libexiv2-devel mingw64-libcurl \
               mingw64-libcurl-devel mingw64-libjpeg mingw64-libjpeg-devel \
               mingw64-libtiff mingw64-libtiff-devel mingw64-liblcms2 \
               mingw64-liblcms2-devel mingw64-libopenjpeg1 mingw64-libopenjpeg-devel \
               mingw64-GraphicsMagick mingw64-GraphicsMagick-devel mingw64-libexpat \
               mingw64-libexpat-devel mingw64-libsoup mingw64-libsoup-devel

# the jpeg headers installed by openSUSE contain some definitions that hurt us so we have to patch these.
# this is super ugly and needs to be redone whenever the library is updated.
sed -e 's:^#define PACKAGE:// #define PACKAGE:g' -i /usr/x86_64-w64-mingw32/sys-root/mingw/include/jconfig.h

------------------ snap ------------------

- Put the sources somewhere, probably by grabbing them from git:
    git clone https://github.com/darktable-org/darktable.git
- cd into the freshly cloned source tree and edit cmake/toolchain_mingw64.cmake if needed,
  it should be ok if using the virtualized openSUSE environment as described here.
- Build darktable using this script (save it as build-win.sh). Make sure to edit it
  to suit your environment, too, if needed. Again, if following these directions it
  should be working out of the box. Run the script from the root directory of
  the git clone (i.e., put it next to build.sh):

------------------ snip ------------------
#!/bin/sh

# Change this to reflect your setup
# Also edit cmake/toolchain_mingw64.cmake
MINGW="/usr/x86_64-w64-mingw32"
RUNTIME_PREFIX=".."


export PATH=${MINGW}/bin:$PATH
export CPATH=${MINGW}/include:${MINGW}/include/OpenEXR/
export LD_LIBRARY_PATH=${MINGW}/lib
export LD_RUN_PATH=${MINGW}/lib
export PKG_CONFIG_LIBDIR=${MINGW}/lib/pkgconfig

DT_SRC_DIR=`dirname "$0"`
DT_SRC_DIR=`cd "$DT_SRC_DIR"; pwd`
BUILD_DIR="build-win"

cd $DT_SRC_DIR;

INSTALL_PREFIX=$1
if [ "$INSTALL_PREFIX" =  "" ]; then
  INSTALL_PREFIX=/opt/darktable-win/
fi

BUILD_TYPE=$2
if [ "$BUILD_TYPE" =  "" ]; then
        BUILD_TYPE=RelWithDebInfo
fi

echo "Installing to $INSTALL_PREFIX for $BUILD_TYPE"
echo "WARNING:"
echo "     This is a highly experimental try to cross compile darktable for Windows!"
echo "     It is not guaranteed to compile or do anything useful when executed!"
echo ""

if [ ! -d ${BUILD_DIR} ]; then
  mkdir ${BUILD_DIR}
fi

cd ${BUILD_DIR}

MAKE_TASKS=1
if [ -r /proc/cpuinfo ]; then
  MAKE_TASKS=$(grep -c "^processor" /proc/cpuinfo)
elif [ -x /sbin/sysctl ]; then
  TMP_CORES=$(/sbin/sysctl -n hw.ncpu 2>/dev/null)
  if [ "$?" = "0" ]; then
    MAKE_TASKS=$TMP_CORES
  fi
fi

if [ "$(($MAKE_TASKS < 1))" -eq 1 ]; then
  MAKE_TASKS=1
fi

cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain_mingw64.cmake \
      -DCMAKE_INSTALL_PREFIX=${INSTALL_PREFIX} \
      -DRUNTIME_INSTALL_PREFIX=${RUNTIME_PREFIX} \
      -DCMAKE_INSTALL_LOCALEDIR=${RUNTIME_PREFIX}/share/locale \
      -DCMAKE_BUILD_TYPE=${BUILD_TYPE} \
      -DUSE_OPENCL=Off \
      -DDONT_USE_INTERNAL_LUA=On \
      .. \
&& make -j $MAKE_TASKS

if [ $? = 0 ]; then
  echo "Darktable finished building, to actually install darktable you need to type:"
  echo "# cd ${BUILD_DIR}; sudo make install"
fi

------------------ snap ------------------

- Last but not least:
   cd build-win; make install
- Now you have a Windows version of darktable in /opt/darktable-win/, which
  should be suited for 64 bit Windows installations. Before you can run it do
    cp /usr/x86_64-w64-mingw32/sys-root/mingw/bin/*.dll /opt/darktable-win/bin/
- Done. Copy /opt/darktable-win/ to a Windows box, go to the bin folder and
  double click darktable.exe.

KNOWN ISSUES:

- The current working directory has to be the bin folder when running the .exe.
  Double clicking in Explorer does that for you.
- GTK engine is not copied over/found currently, probably just a matter of putting
  the right DLLs into the correct places and/or setting some environment variables.
  We also want "wimp" in gtkrc instead of clearlooks. You can find it in
  /usr/x86_64-w64-mingw32/sys-root/mingw/lib/gtk-2.0/2.10.0/engines/
- A stupid error message about missing dbus symbols in a DLL. This is fixed in libglib
  already but the version used by us doesn't have it. Using newer openSUSE releases
  might fix this but I couldn't find lensfun and libexiv2 for them. Some day we might
  add our own build service to OBS so that will go away eventually.
- Moving darktable's main window freezes the application. No idea what's going on
  there. Since I have seen some Windows installations moving the window on startup
  and subsequently freezing automatically it might be impossible to run darktable
  for you.
- Building with openCL isn't completely done yet (links used for caching). This
  might change soonish, no idea if it would work in the end though.
- Importing directories didn't work in my tests (freezes), single images imported
  fine though, I could edit them and export them.
- Lua support doesn't compile right now since it uses select() on a file which is not
  supported on Windows.

Have fun with this, report back your findings and understand that we don't intend
to officially support a Windows version, let alone provide binaries for download.