Fixed how to get GENERATOR_IS_MULTI_CONFIG

`GENERATOR_IS_MULTI_CONFIG` is a global property, not a variable.

CMakeLists.txt

@@ -24,9 +24,10 @@ if(NOT "${CMAKE_BUILD_TYPE}" MATCHES "^(Debug|Release|RelWithDebInfo|MinSizeRel)
   message(FATAL_ERROR "Please re-run CMake, specifying -DCMAKE_BUILD_TYPE=Debug , -DCMAKE_BUILD_TYPE=Release , -DCMAKE_BUILD_TYPE=RelWithDebInfo or -DCMAKE_BUILD_TYPE=MinSizeRel .")
 endif()
 
+get_property(isMultiConfig GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
 option(FRUIT_ALLOW_MULTI_CONFIG "Allow multi-configuration generator. If this option is OFF, enforces that a single configuration is used." OFF)
 if(FRUIT_ALLOW_MULTI_CONFIG)
-  if(GENERATOR_IS_MULTI_CONFIG)
+  if(isMultiConfig)
     message(STATUS "${CMAKE_PROJECT_NAME} supports the CMake multi-configuration generator,"
                    " but is optimized for the `CMAKE_BUILD_TYPE` specified in the CMake configuration step,"
                    " and tests will only work in this configuration.")