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.
Before, namespace packages would break in auditwheel due to a
superfluous check (fixed in the commit referenced in the Dockerfiles).
Now, the auditwheel used in the manylinux1-support Docker images can
handle namespace packages, and we may thus use them. This should
alleviate future user pain w.r.t. installation of grpcio-tools causing
loss of grpcio files and vice versa (e.g. requiring a reinstall of
grpcio following installation of grpcio-tools).
This grossly simplifies the protoc invocation to:
python -m grpc.protoc.compiler --python_out=... --grpc_python_out=...
[...] --plugin=protoc-gen-python-grpc=grpc_python_protoc_plugin... [...]