Add comments on win9x builds

doc/BUILD-win9x.md

@@ -91,9 +91,29 @@ 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
+    --enable-shared=no CC=i386-mingw32-gcc \
+    CFLAGS='-ffunction-sections -fdata-sections'
 ```
 
+### DirectX headers and static libraries
+DirectX can help with hardware acceleration on older hardware. Fortunately,
+the SDL team distribute both source and binaries that can be included when
+building `sdl-1.2.15`. While two tarballs are distributed, only
+`directx-devel.tar.gz` has been tested so far.
+
+[directx-devel.tar.gz](https://www.libsdl.org/extras/win32/common/directx-devel.tar.gz)
+[directx-source.tar.z](https://www.libsdl.org/extras/win32/common/directx-source.tar.gz)
+
+```sh
+../sdl-1.2.15-src/configure --host=i386-mingw32 --prefix=$HOME/sdl-1.2.15 \
+    --enable-shared=no CC=i386-mingw32-gcc \
+    CFLAGS="-ffunction-sections -fdata-sections \
+    -I$HOME/directx-devel/include -L$HOME/directx-devel/lib"
+```
+
+In order to take advantage of DirectX, remember to assign the environment
+variable `SDL_VIDEODRIVER` to `directx` before running `rts`.
+
 ## SDL_mixer
 Only `SDL_mixer-1.2.12` is known to work so far. Since `rts` only uses
 WAVE files, support for other audio formats is not required.
@@ -102,5 +122,6 @@ WAVE files, support for other audio formats is not required.
 ../SDL_mixer-1.2.12-src/configure --host=i386-mingw32 --enable-shared=no \
     --prefix=$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=$HOME/sdl-1.2.15
+    --disable-music-flac CC=i386-mingw32-gcc --with-sdl-prefix=$HOME/sdl-1.2.15 \
+    CFLAGS='-ffunction-sections -fdata-sections'
 ```