cmake: releases have a REVISION.txt instead of VERSION.txt

CMakeLists.txt

@@ -3053,8 +3053,8 @@ endif()
 
 # Compat helpers for the configuration files
 
-if(EXISTS "${PROJECT_SOURCE_DIR}/VERSION.txt")
-  file(READ "${PROJECT_SOURCE_DIR}/VERSION.txt" SDL_SOURCE_VERSION)
+if(EXISTS "${PROJECT_SOURCE_DIR}/REVISION.txt")
+  file(READ "${PROJECT_SOURCE_DIR}/REVISION.txt" SDL_SOURCE_VERSION)
   string(STRIP "${SDL_SOURCE_VERSION}" SDL_SOURCE_VERSION)
 endif()