Resolve "Apple Clang for Xcode 10.0 not supported with MACOSX_DEPLOYMENT_TARGET=10.7"

This PR is split into two commits. The first one adds support for the CMAKE_CXX_STDLIB variable. Dependent on the Xcode version that variable can be left empty to select the compiler default or set to either libc++ or libstdc++. It is then applied to both the Xcode project settings and the architecture flags.

I chose to introduce the CMAKE_CXX_STDLIB setting because I did not know if changing CMAKE_CXX_FLAGS(_INIT) within Modules/Platform/Apple-AppleClang-CXX.cmake would be a good idea. Having an extra variable also ensures that I can forward the setting to try_compile unconditionally whereas the CXX_FLAGS forwarding code seems to be dependent on a policy.

The second commit sets the CMAKE_CXX_STDLIB flag to libc++ if AppleClang >= 10 is detected. Starting with Xcode 10 the GNU libstdc++ C++ runtime has been removed. Deployment targets that would have autoselected libstdc++ with Xcode 9 now emit the error message to migrate to libstdc++ instead of also defaulting to libstdc++.

Please review the changes. If you're OK with the changes I'll add documentation and a test.

Thanks, Gregor

Closes #18396 (closed)