Resolves#357
- Add project property ``grpc.skip.codegen``, which is false by default.
People who don't change the codegen nor the proto files can set it to
true so that they don't need to set up C++ compilation.
- Check in all generated files under ``src/generated``.
- Stop defining custom architectures because VisualCpp toolchain doesn't
allow it. Reference to built-in architectures 'x86' and 'x86_64' since
they are supported by all toolchains.
- Remove 'local_arch' from platforms. For unsupported platform, we just
do not specify the target.
- Target no more than one platform at a time. This simplifies the build
script a lot.
- Remove the TARGET_ARCHS environment variable. Add system property
``arch`` to override ``osdetector.arch``.
- Add ``vc.disable`` to override the default choice of VisualCpp on
Windows.
- Add ``vc.`` prefix to the properties that are only used with VC++
A Gradle protoc plugin is used for generating and compiling the grpc
codegen. The code organization was changed to match what Gradle expects.
Proto 3 is now required.
Kun Zhang