Bug 688681: Stop using .def files in Visual Studio builds

Bug 688681: Stop using .def files in Visual Studio builds

Since we are now starting to use __declspec (dllexport) to export the
public functions during the build of the GLib DLLs (i.e. to generate the
.lib files), we don't want to generate the .def files from the .symbols
files as we did before for a long time.

This removes from the projects the custom build steps to generate the
various .def files

This will also update the pre-configured config.h(.win32.in) to define
_GLIB_EXTERN appropriately as __declspec (dllexport), as well as making its
entries reflect config.h.in more closely.