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. See the CMAKE_MFC_FLAG variable documentation to configure use of MFC for WinMain executables. This property is initialized by the value of the variable CMAKE_WIN32_EXECUTABLE if it is set when a target is created.