Add std=c++11 to CMakeLists for older compilers

[Armagetron]

Compiling the library with older compilers like gcc 4.9 that do not compile the code with the C++11 standard by default may cause errors. Therefore I created this pull request that adds the C++11 standard explicitly.

---

This change is 

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Armagetron]

I signed it!

[googlebot]

CLAs look good, thanks!

[lararennie]

Can you explain what problem this is fixing? We aren't expecting people to use c++11 compilers: are there parts of the code where c++11 has snuck in? You say "may cause errors" -> has it caused errors?

[Armagetron]

I attached a build log I get when I compile the current version (master HEAD) on a Raspberry Pi with Raspbian Jessie and a gcc 4.9.2. With the fix I provided the error is gone.

I think this is caused by the protobuf compiler which generates C++11 code by default.

[lararennie]

Thanks for more context!
Indeed, the latest version seems to require C++11 by default (protocolbuffers/protobuf#597)
However (this patch)[https://github.com/google/protobuf/commit/ffde972ac705e70f597267537b033d7c00ea54c5] seems to turn them off.

We should see if the latest protobuf version still has this problem: it seems they are considering branching and having a c++11 version and a non-c++11 version so they didn't intend to turn it on...

[Armagetron]

I tested the current libphonenumber (1f69a99) together with the current protobuf (protocolbuffers/protobuf@f0a5c10) version with the gcc 4.9 I don't get any errors. So it works without c++11 again.

However if the protobuf team decides to move their future development to a c++11 based version, libphonenumber should probably go the same way so that the newest version can be used. A pre c++11 fork would only receive bug-fix support and on systems that ship the new version by default one would have to manage two versions.

[lararennie]

Great to hear it works again!
It is a good point that the systems shipping the new version would have to manage two versions, but not everyone is ready to move to C++11 either, so we'd have to make such a move carefully and with input from our users.
I'll close this in the meantime as it's not needed right now. Thanks for following up!