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
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 ust be a better way to do this.but can't find any documentation.
Here `TOSTRING` changes version string to literal `VERSION` and `XXHASH_VERSION`. It's meaningless.

I googled and found [this](https://docs.python.org/2/distutils/apiref.html?highlight=define_macros#distutils.core.Extension), we could use ``define_macros``. Would you mind to change the `TOSTRING` way to using `define_macros`.