Correction of C++ Standard Flag from '17' to '1z' for clang 4.0 - Linux Job in azure-pipelines.yml

[gopi487krishna]

Currently the code in boost.Astronomy does not compile successfully on clang 4.0 due to the wrong value of CXXSTD flag being passed to the compiler. Kindly change the value from 17 to 1z for the compilation to be successful on clang 4.0

Current value: azure-pipelines.yml

 Clang 4:
          B2_TOOLSET: clang
          B2_CXXSTD: 14,17
          CXX: clang++-4.0
          PACKAGES: clang-4.0
          LLVM_REPO: llvm-toolchain-xenial-4.0

Correct value : azure-pipelines.yml

 Clang 4:
          B2_TOOLSET: clang
          B2_CXXSTD: 14,1z
          CXX: clang++-4.0
          PACKAGES: clang-4.0
          LLVM_REPO: llvm-toolchain-xenial-4.0

[rjnrohit]

Also, configuration for test Mac os needs to be updated. These checks are getting cancelled automatically.