README.md: Update documentation for Win9x builds

README.md

@@ -88,13 +88,42 @@ on real hardware.
 ```sh
 mkdir build
 cd build
-cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/win9x-toolchain.cmake
+SDLDIR=<sdl-prefix> \
+    SDLMIXERDIR=<sdl_mixer-prefix> \
+    SDLGFXDIR=<sdl_gfx-prefix> \
+    cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/win9x-toolchain.cmake
 make -j$(nproc --all)
 ```
 
+Where:
+
+- `SDLDIR` is the path to the cross-compiled version for `SDL-1.2`, which
+would correspond to `./configure --prefix=$SDLDIR` used when building
+`SDL-1.2`.
+- `SDLMIXERDIR` is the path to the cross-compiled version for `SDL_mixer`,
+which would correspond to `./configure --prefix=$SDLMIXERDIR` used in
+building `SDL_mixer`.
+- `SDLGFXDIR` is the path to the cross-compiled version for `SDL_gfx`,
+which would correspond to `./configure --prefix=$SDLGFXDIR` used in
+building `SDL_gfx`.
+
 A stripped version of the executable, as well as game assets, will be
 located in `build/cdimg`.
 
+#### Dependencies
+
+A cross-compiled `i386-mingw32` version of all required dependencies
+is needed before building `rts`.
+
+Note upstream CMake does not provide `FindSDL_gfx.cmake` as of the time
+of this writing, so it is provisionally provided on
+[this repository](cmake/FindSDL_gfx.cmake).
+
+[Ongoing upstream PR](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/7475)
+
+Read [the documentation](doc/BUILD-win9x.md) for further reference on
+how to build the dependencies from source.
+
 ## License
 
 Unless stated otherwise, **RTS** follows the license described by the