dos: get SDL via CMake

Now that the DOS SDL supports CMake, this is a much better option that
`configure`, because it lets us easily test out local changes to SDL,
e.g.:

```bash
cmake -S. -Bbuild-dos -DCMAKE_TOOLCHAIN_FILE=CMake/platforms/djcpp.toolchain.cmake \
  -DTARGET_PLATFORM="dos" -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTING=OFF \
  -DFETCHCONTENT_SOURCE_DIR_SDL2="${HOME}/diasurgical-SDL"
```

3rdParty/SDL2/CMakeLists.txt

@@ -14,8 +14,18 @@ set(SDL_TEST_ENABLED_BY_DEFAULT OFF)
 
 include(functions/FetchContent_ExcludeFromAll_backport)
 include(FetchContent)
+
+if(TARGET_PLATFORM STREQUAL "dos")
+  set(DOS ON)
+  FetchContent_Declare(SDL2
+    # branch: dos-vbe-rebase
+    URL https://github.com/diasurgical/SDL/archive/6a87741b50e52b93852eb8ab59a89495ae9e607b.tar.gz
+    URL_HASH MD5=a41f10f31b3ec00256c85131291bb0c3
+  )
+else()
   FetchContent_Declare(SDL2
     URL https://github.com/libsdl-org/SDL/releases/download/release-2.32.8/SDL2-2.32.8.tar.gz
     URL_HASH SHA256=0ca83e9c9b31e18288c7ec811108e58bac1f1bb5ec6577ad386830eac51c787e
   )
+endif()
 FetchContent_MakeAvailable_ExcludeFromAll(SDL2)

CMake/platforms/dos.cmake

@@ -11,7 +11,7 @@ set(DEVILUTIONX_SYSTEM_BZIP2 OFF)
 set(DEVILUTIONX_SYSTEM_LIBFMT OFF)
 set(DEVILUTIONX_SYSTEM_ZLIB OFF)
 set(DEVILUTIONX_STATIC_ZLIB ON)
-set(DEVILUTIONX_SYSTEM_SDL2 ON)
+set(DEVILUTIONX_SYSTEM_SDL2 OFF)
 set(DEVILUTIONX_STATIC_SDL2 ON)
 set(DEVILUTIONX_SYSTEM_SDL_IMAGE OFF)
 set(DEVILUTIONX_SYSTEM_LIBPNG OFF)

Packaging/windows/dos-prep.sh

@@ -31,25 +31,12 @@ build_and_install_djgpp() {
 	$SUDO rm -rf build-gcc
 }
 
-# Build and install SDL2 for DOS
-build_and_install_sdl2() {
-	git clone --branch=dos-vbe-rebase https://github.com/diasurgical/SDL.git
-	cd SDL
-	"${INSTALL_PREFIX}/bin/i386-pc-msdosdjgpp-setenv" autoreconf -fi --verbose
-	"${INSTALL_PREFIX}/bin/i386-pc-msdosdjgpp-setenv" ./configure --host=i386-pc-msdosdjgpp --prefix="${INSTALL_PREFIX}/i386-pc-msdosdjgpp" --disable-shared --enable-static --enable-video-svga --enable-timer-dos --enable-uclock
-	"${INSTALL_PREFIX}/bin/i386-pc-msdosdjgpp-setenv" make -j$(nproc)
-	$SUDO "${INSTALL_PREFIX}/bin/i386-pc-msdosdjgpp-setenv" make install
-	cd -
-	rm -rf SDL
-}
-
 main() {
 	set -x
 	mkdir -p tmp/dos-prep
 	cd tmp/dos-prep
 	install_system_deps
 	build_and_install_djgpp
-	build_and_install_sdl2
 }
 
 main "$@"