Fix VSProject generation issues with SConstruct:
* Always describe the Visual Studio targets
* Prevent checking nonexistant enviro vars
* Prevent pkg-config protobuf env vars if not clang/gcc
* Don't use any pkg-config enviro vars unless clang/gcc
* Make all VSProject/config directories windows-style
* Prevent beastobjc.mm from showing up in vcxproj
* Remove duplicate \src\protobuf\src
* Consistent quoting