Allows scons vcxproj to work on all systems

Changes along the way:
* Makes itemlist work in Python 3
* Makes the toolchain loop more straightforward
* Removes an old variable
* Prevents checking nonexistant enviro vars
* Prevents pkg-config protobuf env vars if not clang/gcc
* Removes tabs
* Doesn't use any pkg-config enviro vars unless clang/gcc