Fix usage of global property GENERATOR_IS_MULTI_CONFIG

CMakeLists.txt

@@ -43,7 +43,8 @@ endif ()
 
 # LoadStaticSharedTargets is a CMake script, which we never want to debug
 # If using a multi config generator
-if (GENERATOR_IS_MULTI_CONFIG)
+get_property(_IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+if (_IS_MULTI_CONFIG)
     set(CMAKE_CONFIGURATION_TYPES "Release")
     set(CMAKE_DEFAULT_BUILD_TYPE "Release")
 # If using a single config generator