Build an executable with a WinMain entry point on windows.
When this property is set to true the executable when linked on
Windows will be created with a WinMain() entry point instead of just
main(). This makes it a GUI executable instead of a console application.
CMAKE_MFC_FLAG variable documentation to
configure use of the Microsoft Foundation Classes (MFC) for WinMain
executables. This property is initialized by the value of the
CMAKE_WIN32_EXECUTABLE variable if it is set when
a target is created.
This property supports
generator expressions, except if the
target is managed (contains C# code.)