Bug 688681: Stop using the .def file for GThread Visual C++ builds

Bug 688681: Stop using the .def file for GThread Visual C++ builds

We should also stop using gthread/gthread.def file as well, since we
also use __declspec (dllexport) for the two (deprecated) functions there
since commit f8756694.

This should also silence some linker warnings in x64 builds.

Also get rid of the references to the .symbols files in the
.vcxproj.filters(in) in the various Visual C++ 2010 projects