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

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...