What's the equivalent of gcc's -mwindows option in cmake?

I'm following the tuto:

http://zetcode.com/tutorials/gtktutorial/firstprograms/

It works but each time I double click on the executable,there is a console which I don't want it there.

How do I get rid of that console?

I tried this:

add_executable(Cmd WIN32 cmd.c)

But got this fatal error:

MSVCRTD.lib(crtexew.obj) : error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartup
Cmd.exe : fatal error LNK1120: 1 unresolved externals

While using gcc directly works:

gcc -o Cmd cmd.c -mwindows ..

I'm guessing it has something to do with the entry function: int main( int argc, char *argv[]),but why gcc works?

How can I make it work with cmake?

UPDATE

Let me paste the source code here for convenience:

#include <gtk/gtk.h>

int main( int argc, char *argv[])
{
  GtkWidget *window;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_widget_show(window);

  gtk_main();

  return 0;
}

UPDATE2

Why gcc -mwindows works but add_executable(Cmd WIN32 cmd.c) not?

Maybe that's not the equivalent for -mwindows in cmake?