Update cmake for SignalR C++ client on Mac OS X

[wegylexy]

Summary of the changes

• Skip gtest
• Patch Boost and OpenSSL in /usr/local/opt

brew install cpprestsdk boost openssl
cmake .. -DCMAKE_CXX_FLAGS=-I/usr/local/opt/openssl/include -DCPPREST_INCLUDE_DIR=/usr/local/opt/cpprestsdk/include -DCPPREST_LIB_DIR=/usr/local/opt/cpprestsdk/lib
make

Addresses #8066

[dnfclas]

All CLA requirements met.

[BrennanConroy]

Thanks for your interest in the C++ client!

Unfortunately we aren't ready to take CMake changes yet.

We need to setup our CI builds to build and test the client on the different OS's first so we don't accidentally break the build files. And we'll want more than just macOS working.

We have already started looking into making CMake work cross-plat, you can see the WIP changes in the branch https://github.com/aspnet/AspNetCore/tree/brecon/cmake

I've filed #8706 so we can track and discuss things there if you'd like.

[wegylexy]

It already works on Ubuntu before this change. Mac just needs more help to find Boost and OpenSSL. For Windows, we have Visual Studio. So after this change, all 3 platforms can build.

[analogrelay]

Thanks for the PR, but we have some build process stuff to integrate with (version numbers to align with and MSBuild goop to connect it up to). As @BrennanConroy said, we're working on getting this in soon. I'm going to close this since the changes we've already got will end up conflicting with this.