Podsvirov CMake changes backported to 3.0.x

[rpavlik]

These are CMake changes, including an essential bugfix for protobuf_MODULE_COMPATIBLE, originally found in #1865 by @podsvirov - all I did was a git rebase --onto to get them on the 3.0.x branch.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[bazel-io]

Can one of the admins verify this patch?

[googlebot]

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[rpavlik]

I previously had the CLA signed (via employer) - I have an invite that I accepted back on august 3, 2015 to authorized-google-contributors. I suspect the fact that we switched to Google Apps, err, GSuite, since then might be causing the confusion with the CLA verification. (The google account I previously was using with this email prior to migration is now only identified as rpavliksensics a t gmail, and I can see the group on that account...)

[edit: CLA re-signed, new group is google-cla on the domain sensics.com]

[acozzette]

ok to test

[acozzette]

retest this please (talking to the Jenkins bot)

[acozzette]

@rpavlik Actually I believe we are no longer really maintaining the 3.0.x branch and we likely won't be making any more releases in the 3.0.x series. In that case is there any particular reason you need to backport these fixes?

[rpavlik]

I signed it!

[rpavlik]

I wasn't sure what the branch maintenance policy was - it didn't appear to be laid out clearly anywhere (nor did anything newer than 3.0.0 show up on Google Developers page when I checked), and I assumed the presence of a 3.0.x branch, in the absence of other guidance, was then a stable release. Fine by me if no more 3.0.x releases occur, (I've actually switched to 3.1.x in the meantime) but then it probably should be documented somewhere :D (release notes of the downloads in GitHub would be one place to start...)