This fixes an issue that prevented bitcoind to cross build.
CMake analyses the compiler to extract a library path, which conflicts
with the path from the depends/ directory.
This issue depends on the installed compiler, so it may not appear on
every system. I got the issue with Ubuntu 18.04 (WSL).

Prerequisite:
Follow the doc/build-windows.md and depends/README.md documentation
and install the dependencies for i686-w64-mingw32 and
x86_64-w64-mingw32.

# 32-bit version
mkdir buildcmake && cd buildcmake
cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake
ninja

Check the build succeeds.

To run the generated src/bitcoind.exe binary some dll files need to be
copied. The files are moved to a target repository <dir>. The
installed i686-w64-mingw32 version number is <mingw-ver>. The
version number can be obtained with i686-w64-mingw32-c++ --version
(7.3 on my WSL Ubuntu 18.04).

mkdir -p <dir>
cp src/bitcoind.exe <dir>
cp /usr/i686-w64-mingw32/lib/libwinpthread-1.dll <dir>
cp /usr/lib/gcc/i686-w64-mingw32/<mingw-ver>-posix/libstdc++-6.dll <dir>
cp /usr/lib/gcc/i686-w64-mingw32/<mingw-ver>-posix/libgcc_s_sjlj-1.dll
<dir>

Operate the same for Win64.

# 64-bit version
rm -rf *
cmake -GNinja .. -DBUILD_BITCOIN_SEEDER=OFF \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake
ninja

Check the build succeeds.
To run the generated src/bitcoind.exe binary some dll files need to be
copied. The files are moved to a target repository <dir>. The
installed x86_64-w64-mingw32 version number is <mingw-ver>. The
version number can be obtained with x86_64-w64-mingw32-c++ --version
(7.3 on my WSL Ubuntu 18.04).

mkdir -p <dir>
cp src/bitcoind.exe <dir>
cp /usr/x86_64-w64-mingw32/lib/libwinpthread-1.dll <dir>
cp /usr/lib/gcc/x86_64-w64-mingw32/<mingw-ver>-posix/libstdc++-6.dll
<dir>
cp /usr/lib/gcc/x86_64-w64-mingw32/<mingw-ver>-posix/libgcc_s_seh-1.dll
<dir>