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.
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