Enable google/cloud/channel on macOS

[devjgm]

https://github.com/googleapis/google-cloud-cpp/pull/8116 adds the google/cloud/channel library. However, that build breaks on macos due to:

bazel-out/darwin-fastbuild/bin/external/com_google_googleapis/google/cloud/channel/v1/common.pb.h:545:33: error: expected member name or ';' after declaration specifiers
  static constexpr CustomerType DOMAIN =
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/Applications/Xcode_12.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include/math.h:732:17: note: expanded from macro 'DOMAIN'
#define DOMAIN          1
                        ^

https://source.cloud.google.com/results/invocations/8ee5da3b-6410-4185-a0d2-b38f5282069c/targets/cloud-cpp%2Fgithub%2Fgoogle-cloud-cpp%2Fmain%2Fmacos%2Fbazel-presubmit/log

That is, protobuf-generated code uses a symbol named DOMAIN that conflicts w/ a macro definition.

We will temporarily disable this library on macos, and re-enable it once protobuf is fixed to not choke on DOMAIN.

Originally posted by @devjgm in https://github.com/googleapis/google-cloud-cpp/issues/8115#issuecomment-1022677732

[devjgm]

The fix was submitted internally in cl/424720867. Now we need to wait for a release that contains the fix.

[coryan]

This is also a problem on Windows.

[coryan]

I sent a CL to fix Protobuf, but it will take a while to seep into the open source version.

[coryan]

Still blocked on GCC < 8. I will send a PR to Protobuf, since we have about a year left with GCC 7.

[coryan]

This is now fixed.