Add build instructions for Win9x

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+# Cross-compilation for Win9x
+Creating a toolchain targetting `i386-mingw32` is not usually possible
+with the distribution-provided packages. This documentation describes
+which versions are known to work so far, as well as detailed
+configuration scripts.
+
+The following documentation assumes the installation prefix
+`$HOME/i386-mingw32` for the GNU toolchain. However, feel free to
+change this value, if required.
+
+Some specific library versions might be hard to find, so download links
+are provided.
+
+## Environment variables
+Since it is desirable to avoid messing with system-level libraries,
+when building for Win9x, `rts` will look for dependencies by inspecting
+the following environment variables:
+
+- `SDL_PATH`
+- `SDL_TTF_PATH`
+- `FREETYPE_PATH`
+- `SDL_MIXER_PATH`
+
+Also, the directory containing the GNU toolchain binaries (which
+should be located at `$HOME/i386-mingw32/bin` by default) must be added
+to the `PATH` variable.
+
+## gcc
+From own experience, modern versions of `gcc` cannot be used to compile
+a cross-compiler for Win9x. Instead, a *much* older version is required.
+So far, `gcc-3.4.5` is the latest known version to work.
+
+### Known issues
+- Take into account only C99 is supported by `gcc-3.4.5`.
+- `gcc-3.4.5` complains about the use of `-ffunction-sections` and
+`-fdata-sections`.
+
+```sh
+../gcc-3.4.5-20060117-2/configure --disable-nls --disable-rpath \
+    --disable-win32-registry --enable-languages=c --enable-sjlj-exceptions \
+    --enable-threads=win32 --prefix=$HOME/i386-mingw32 \
+    --program-prefix=i386-mingw32- --target=i386 -mingw32 --with-gnu-as \
+    --with-gnu-ld
+```
+
+[gcc-core-3.4.5-20060117-2-src.tar.gz](https://sourceforge.net/proejcts/mingw/files/MinGW/Base/gcc/Version3/Previous%20Release_%20gcc-3.45-20060117-2/gcc-core-3.4.5-20060117-2-src.tar.gz)
+
+## binutils
+GNU binutils is usually not so problematic, so any version should do.
+Only `binutils-2.35` has been tested.
+
+```sh
+../binutils-2.35/configure --prefix=$HOME/i386-mingw32 --target=i386-mingw32
+```
+## mingwrt and w32api
+So far, only versions `mingwrt-3.15.2-mingw32` and `w32api-3.13-mingw32`
+have been tested.
+
+### Known issues
+- Some hacks have to be done in order to compile these libraries. These
+are described on the script below.
+
+```sh
+../w32api-3.13-mingw32/configure --prefix=$HOME/i386-mingw32 --host=i386-mingw32
+# Hack at line below
+cd mingwrt-3.15.2-mingw32/ && cp -r include/* ~/i386-mingw32/include/
+../mingwrt-3.15.2-mingw32/configure --prefix=$HOME/i386-mingw32 --target=i386-mingw32
+../sdl-1.2.15-src/configure --host=i386-mingw32 --prefix=/home/xavier/sdl-1.2.15 \
+    --enable-shared=no CC=i386-mingw32-gcc
+```
+
+[mingw-runtime-3.15.2](https://sourceforge.net/projects/mingw/files/OldFiles/mingwrt-3.15.2/mingwrt-3.15.2-mingw32-src.tar.gz)
+
+[mingw-w32api-3.13](https://sourceforge.net/projects/mingw/files/OldFiles/w32api-3.13/w32api-3.13-mingw32-src.tar.gz)
+
+## freetype (not required)
+Only `freetype-2.4.8` is known to work so far.
+
+### Known issues
+- Apparently, out of source builds are not possible.
+
+```sh
+cd freetype-2.4.8-src/ && ./configure --prefix=$HOME/freetype-2.4.8 \
+        --host=i386-mingw32 --enable-shared=no CC=i386-mingw32-gcc
+```
+
+## SDL
+`sdl-1.2.15` is the latest version supported version for the Win9x family
+of operating systems. In order to keep things easy, the script below
+configures SDL as a static library.
+
+```sh
+../sdl-1.2.15-src/configure --host=i386-mingw32 --prefix=$HOME/sdl-1.2.15 \
+    --enable-shared=no CC=i386-mingw32-gcc
+```
+
+## SDL_mixer
+Only `SDL_mixer-1.2.12` is known to work so far. Since `rts` only uses
+WAVE files, support for other audio formats is not required.
+
+```sh
+../SDL_mixer-1.2.12-src/configure --host=i386-mingw32 --enable-shared=no \
+    --prefix=$HOME/SDL_mixer-1.2.12 --disable-music-cmd --disable-music-mod \
+    --disable-music-ogg --disable-music-mp3 --disable-music-midi \
+    --disable-music-flac CC=i386-mingw32-gcc --with-sdl-prefix=$HOME/sdl-1.2.15
+```