Activated building SDL2 directly from submodule on Unix as well

.travis.yml

@@ -39,9 +39,6 @@ before_install:
     fi
 # Building for SDL here
   - if [ "$BUILD_SDL" == "YES" ]; then
-      curl -O https://www.libsdl.org/release/SDL2-2.0.3.tar.gz &&
-      tar -xzvf SDL2-2.0.3.tar.gz &&
-      mv SDL2-2.0.3 thirdparty/SDL2 &&
       export SDL2DIR="$TRAVIS_BUILD_DIR/thirdparty/SDL2";
     fi
 

CMakeLists.txt

@@ -99,7 +99,7 @@ WAGIC_OPTION(BUILD_PNG     "build png from source"     (WIN32 OR APPLE OR PSP OR
 WAGIC_OPTION(BUILD_UNZIP   "build unzip from source"   ON)
 WAGIC_OPTION(BUILD_TINYXML "build tinyxml from source" (WIN32 OR APPLE OR PSP OR ANDROID OR EMSCRIPTEN))
 WAGIC_OPTION(BUILD_ZIPFS   "build zipfs from source"   ON)
-WAGIC_OPTION(BUILD_SDL2    "build SDL2 from source"    (backend_sdl AND (WIN32 OR ANDROID OR EMSCRIPTEN)))
+WAGIC_OPTION(BUILD_SDL2    "build SDL2 from source"    (backend_sdl AND (UNIX OR WIN32 OR ANDROID OR EMSCRIPTEN)))
 
 #project options
 if(ANDROID)