* Added testing with std=c++17 for latest compilers
Added running latest compilers with -std=c++17 option to CI the test matrix,
Updated cmake configuration to allow passing c++ standard on the command line.
* attempt to fix appveyor break
* added clang6.0, removed c++17 tests for clang 5.0
* commented out tests for clang with c++17 die to issue #695
* Addresed comments