Visual Studio Builds: Don't Generate a .def File

Clutter, like GTK+ and GLib, has recently switched to a visibility-based
method of exporting symbols, so update the Visual Studio build files to
do likewise, by using __declspec (dllexport). This eliminats the need to
use a .def file to export the symbols.  The pre-configured
config.h.win32.in is also updated accordingly for this purpose.  The
clutter.symbols file can be dropped if it is not being used otherwise.