Removes MinGW flags from various build scripts, since they're now set in
our `setup.py`s by default. Also removes the arguments for the
extensions in grpcio's setup.py; that function was already accessing
globals anyway, might as well go whole-hog.
The default compiler for Python c extensions is VS2008
or VS2010 depending on the Python version. Since c-core
moved onto the c99 standard, these compilers are not
compatible.
murgatroid99