Cross compiling notes for Win32 target

docs/build_notes_cross.md

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+Cross compilation notes
+=======================
+
+Static 64 bit windows binary on Ubuntu 15.10 (Wily Werewolf)
+---------------------------------------------------------------------
+
+Install cross compiler and friends
+```sh
+sudo apt-get install g++-mingw-w64-x86-64
+```
+Default is to use Win32 threading model which lacks std::mutex and such. So we change defaults
+```sh
+sudo update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix
+```
+From now on we assume we have everything in `~/dev/`. Get Boost sources unpacked into `~/dev/boost_1_60_0/`
+and change directory to it.
+Now add out cross compiler configuration. Warning: the following will wipe out whatever you had in there.
+```sh
+echo "using gcc : mingw : x86_64-w64-mingw32-g++ ;" > ~/user-config.jam
+```
+Proceed with building Boost normal way, but let's define dedicated staging directory
+```sh
+./bootstrap.sh
+./b2 toolset=gcc-mingw target-os=windows variant=release link=static runtime-link=static address-model=64 \
+  --build-type=minimal --with-filesystem --with-program_options --with-regex --with-date_time \
+  --stagedir=stage-mingw-64
+cd ..
+```
+Now we get & build OpenSSL
+```sh
+git clone https://github.com/openssl/openssl
+cd openssl
+git checkout OpenSSL_1_0_2g
+./Configure mingw64 no-rc2 no-rc4 no-rc5 no-idea no-bf no-cast no-whirlpool no-md2 no-md4 no-ripemd no-mdc2 \
+  no-camellia no-seed no-comp no-krb5 no-gmp no-rfc3779 no-ec2m no-ssl2 no-jpake no-srp no-sctp no-srtp \
+  --prefix=~/dev/stage --cross-compile-prefix=x86_64-w64-mingw32-
+make depend
+make
+make install
+cd ..
+```
+and Zlib
+```sh
+git clone https://github.com/madler/zlib
+cd zlib
+git checkout v1.2.8
+CC=x86_64-w64-mingw32-gcc CFLAGS=-O3 ./configure --static --64 --prefix=~/dev/stage
+make
+make install
+cd ..
+```
+Now we prepare cross toolchain hint file for CMake, let's name it `~/dev/toolchain-mingw.cmake`
+```cmake
+SET(CMAKE_SYSTEM_NAME Windows)
+SET(CMAKE_C_COMPILER x86_64-w64-mingw32-gcc)
+SET(CMAKE_CXX_COMPILER x86_64-w64-mingw32-g++)
+SET(CMAKE_RC_COMPILER x86_64-w64-mingw32-windres)
+SET(CMAKE_FIND_ROOT_PATH /usr/x86_64-w64-mingw32)
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+```
+Download miniupnpc, unpack, and symlink it into `~/dev/miniupnpc/`.
+Finally, we can build i2pd with all that goodness
+```sh
+git clone https://github.com/PurpleI2P/i2pd
+mkdir i2pd-mingw-64-build
+cd i2pd-mingw-64-build
+BOOST_ROOT=~/dev/boost_1_60_0 cmake -G 'Unix Makefiles' ~/dev/i2pd/build -DBUILD_TYPE=Release \
+  -DCMAKE_TOOLCHAIN_FILE=~/dev/toolchain-mingw.cmake -DWITH_AESNI=ON -DWITH_UPNP=ON -DWITH_STATIC=ON \
+  -DWITH_HARDENING=ON -DCMAKE_INSTALL_PREFIX:PATH=~/dev/i2pd-mingw-64-static \
+  -DZLIB_ROOT=~/dev/stage -DBOOST_LIBRARYDIR:PATH=~/dev/boost_1_60_0/stage-mingw-64/lib \
+  -DOPENSSL_ROOT_DIR:PATH=~/dev/stage
+make
+x86_64-w64-mingw32-strip i2pd.exe
+```
+By now, you should have a release build with stripped symbols.