Use CMAKE_CURRENT_BINARY_DIR instead of CMAKE_BINARY_DIR

This allows for building with cmake's add_subdirectory() function.

CMakeLists.txt

@@ -1,4 +1,4 @@
-if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
+if(${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
   message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the SDL source code and call cmake from there")
 endif()