Visual C Compiler compatibility

Visual C won't compile as the setup file uses gcc specific command line
arguments and some gcc constructs are not supported in microsoft's
compiler.

python-xxhash.c:
- Python type initialization requires NULL to break circular reference
- Compiler command line preprocessor definition needs to be explicitely
converted to string

setup.py:
- now uses os.name to determine the platform type

this last change does not check for the compiler itself, but for the platform.  There must be a better way to do this.but can't find any documentation.