Disable linker warning 4197 on Win64, that is caused by the fact that we export

symbols both using __declspec(dllexport) (via the PGDLLIMPORT macro) and using
full-dll-export. This works without warning on Win32, but not on Win64.

In passing, fix the fact that the framework could never deal with more than
one disbled linker warning - because MSVC wants commas between linker warnings,
and semicolons between compiler warnings...