Install protobuf from cmake project

[podsvirov]

Additional export as "protobuf" package
for importing from other cmake projects

Testing binaries in draft pre-release:
https://github.com/podsvirov/protobuf/releases/tag/v3.0.0-5500b0

Importing example avaliable there:
https://github.com/podsvirov/impprotobuf

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[podsvirov]

I signed it!

[googlebot]

CLAs look good, thanks!

[xfxyjwf]

Extract the version number from configure.ac instead?
https://github.com/google/protobuf/blob/master/configure.ac#L15

common.h only has version number like 3000000, but we probably want something like 3.0.0-alpha-4.

[xfxyjwf]

Thanks very much for the change! I was thinking adding installation support but didn't know how to do that.

Also does this PR fix #578?

[podsvirov]

Hi xfxyjwf!

I still do not know much protobuf structure of the project, but not bad versed in CMake.
About half a year ago, I, too, tried to port to CMake protobuf but the project stalled.

On account configure.as think you are right and it is better to place to parse the version of the project. If I understand correctly, but this is the main entry point.

Initially, in the version I was looking for just a number. CMake using 4 numbers (MAYOR.MINOR.PATCH.TWEAK).

However, additional information may also be useful.

To CMake project and export the package need only numbers.

I'll get a modification of the algorithm search numbers from configure.ac.

Of course all this can not affect the module FindProtobuf.

There are still questions arise naming conventions ... How best to call
package. Large or small letters are used, and so on.

[xfxyjwf]

Hi @podsvirov, sorry for the late response. I haven't got time to try the added script on each of the supported platforms but the change itself looks good to me. It seems this PR also exposes some public functions for users to use (PROTOBUF_GENERATE_CPP?). Could you help add some documentation in the cmake/README.md file for it? I'm merging this PR as I don't want to block you further. Thanks again for helping with this change!

[podsvirov]

Excellent! The beginning of a great friendship :-)And this means that the front is big but interesting work!Of course, that these changes should be reflected in cmake/README.md, but there are still a few comments.So other developers could use Protobuf in your projects running CMakewe should take care about creating a "Relocatable Package". Now this is not always the case.
I have questions regarding the use of ZLib:
(a) something wrong with the search package for ZLib MSVC. I see not a standard behavior. Don't need to do.
(b) When linking ZLib needs to use the imported target ZLIB::ZLIB.
If there are no objections, I soon will be doing it and create another PR.
Plans for future developments:
On the basis of this work may forgive in the world following topics on which I work:
https://github.com/podsvirov/grpc/tree/topic-cmake-project
https://github.com/podsvirov/grpc-common/tree/topic-cmake-project
Then the CMake community will increasingly use a bunch CMake-Protobuf-gRPC!