[SDL2] Fix using of DXSDK_DIR in older CMake

- Remove duplicate 'dollar' symbol in the path
- Use command 'file' to normalize file paths

(cherry picked from commit 40b1eac)

Author: bibendovsky

CMakeLists.txt

@@ -1829,8 +1829,10 @@ elseif(WINDOWS)
       set(HAVE_DIRECTX TRUE)
       if(NOT MINGW AND NOT USE_WINSDK_DIRECTX)
       # TODO: change $ENV{DXSDL_DIR} to get the path from the include checks
-        target_link_directories(sdl-build-options INTERFACE "$$ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH}")
-        target_include_directories(sdl-build-options INTERFACE "$ENV{DXSDK_DIR}\\Include")
+        file(TO_CMAKE_PATH "$ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH}" SDL2_TMP_DXSDK_LIB_DIR)
+        target_link_directories(sdl-build-options INTERFACE "${SDL2_TMP_DXSDK_LIB_DIR}")
+        file(TO_CMAKE_PATH "$ENV{DXSDK_DIR}\\Include" SDL2_TMP_DXSDK_INCLUDE_DIR)
+        target_include_directories(sdl-build-options INTERFACE "${SDL2_TMP_DXSDK_INCLUDE_DIR}")
       endif()
     endif()
     set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})