Fix latest GTK master on Visual Studio

Hi,

This attempts to fix the latest GTK master building on Visual Studio, namely by:

• Including stdlib.h in the demo sources where needed. This avoids C4013 errors (aka implicit definition of ... warnings).
• Making sure that the various modules do not try to use G_MODULE_EXPORT in the sources where g_io_module_load(), g_io_module_unload() and g_io_module_query() are defined, because they have been marked with _GLIB_EXTERN (meaning extern by default) in the GIO headers, which Visual Studio does not allow. To export these symbols when building the modules, just define _GLIB_EXTERN as _GDK_EXTERN, which should do the job for for us, since the module sources always included config.h.

With blessings, thank you!