Use CMAKE_CURRENT_BINARY_DIR instead of CMAKE_BINARY_DIR This allows for building with cmake's add_subdirectory() function.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6902207..7b95a4e 100644
--- a/CMakeLists.txt
+++ b/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()