SDL: Use CMAKE_CURRENT_BINARY_DIR instead of CMAKE_BINARY_DIR

SDL Commits
1 
From e52733af8d366146c88b64287667c5e7bd25f776 Mon Sep 17 00:00:00 2001
From: h5p9sl <[EMAIL REDACTED]>
Date: Fri, 9 Jul 2021 13:39:05 -0600
Subject: [PATCH] Use CMAKE_CURRENT_BINARY_DIR instead of CMAKE_BINARY_DIR

This allows for building with cmake's add_subdirectory() function.
---
 CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 690220714..7b95a4e16 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()