Don't build SDL as a static library by default

Linking with SDL dynamically allows end users to update SDL with bug fixes and new controller support and is recommended. This change makes it so SDL is dynamic by default and developers can build SDL themselves with a static option if that's something they need.

CMakeLists.txt

@@ -328,23 +328,14 @@ if(VITA OR PSP OR PS2 OR N3DS)
   set(SDL_LOADSO_ENABLED_BY_DEFAULT OFF)
 endif()
 
-# When defined, respect CMake's BUILD_SHARED_LIBS setting:
-set(SDL_STATIC_ENABLED_BY_DEFAULT ON)
+# Default to just building the shared library
 if (NOT DEFINED SDL_SHARED_ENABLED_BY_DEFAULT)
-  # ...unless decided already (as for EMSCRIPTEN)
-
-  set(SDL_SHARED_ENABLED_BY_DEFAULT OFF)
-
-  if (NOT DEFINED BUILD_SHARED_LIBS)
-    # No preference? Build both, just like the AC/AM configure
-    set(SDL_SHARED_ENABLED_BY_DEFAULT ON)
-
-  elseif (BUILD_SHARED_LIBS)
-    # In this case, we assume the user wants a shared lib and don't build
-    # the static one
-    set(SDL_SHARED_ENABLED_BY_DEFAULT ON)
-    set(SDL_STATIC_ENABLED_BY_DEFAULT OFF)
-  endif()
+  set(SDL_SHARED_ENABLED_BY_DEFAULT ON)
+endif()
+if(SDL_SHARED_ENABLED_BY_DEFAULT)
+  set(SDL_STATIC_ENABLED_BY_DEFAULT OFF)
+else()
+  set(SDL_STATIC_ENABLED_BY_DEFAULT ON)
 endif()
 
 if (NOT DEFINED SDL_TEST_ENABLED_BY_DEFAULT)