CMake: Check for multi-config generator is wrong

[DenizThatMenace]

In CMakeLists.txt you are checking if a multi-config generator is present, by examining a variable GENERATOR_IS_MULTI_CONFIG. (See: https://github.com/zlib-ng/zlib-ng/blob/develop/CMakeLists.txt#L60)

However, that is not how this works. There is no variable GENERATOR_IS_MULTI_CONFIG defined by CMake by default.
Instead, there is a global CMake property of that name defined. (See: CMake documentation
And a property first has to be retrieved and stored into a variable before it can be further examined.

So you need to add the following code before the line where you check the value of the variable GENERATOR_IS_MULTI_CONFIG:

get_property(GENERATOR_IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)

Thereby you are retrieving the value of the global CMake property GENERATOR_IS_MULTI_CONFIG and store it in a local variable of the same name.
Then the following code lines work as expected.