Set CXX standard using target properties

[biddisco]

This is a better (more cmake way) way of setting the cxx standard that using CXX compiler flags directly

[biddisco]

note that there are some places in the CI where the cxx standard is set by hand and I have not modified them in this PR because I was not sure what the CI system does to run the tests, but these should be adjusted to make sure the LIBCDS_CXX_STANDARD flag is set as well before this PR is merged.

build/CI/VASEx-CI-2/cds-libs:gcc-6-cxxflags:       -march=native  -std=c++14
build/CI/VASEx-CI-2/cds-libs:gcc-7-cxxflags:       -march=native  -std=c++1z
build/CI/VASEx-CI-2/cds-libs:gcc-8-cxxflags:       -march=native  -std=c++17 -Wmultistatement-macros
build/CI/VASEx-CI-2/cds-libs:clang-4-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++14 
build/CI/VASEx-CI-2/cds-libs:clang-5-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++1z
build/CI/VASEx-CI-2/cds-libs:clang-6-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++17
build/CI/VASEx-CI-2/cds-libs:clang-7-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++17
build/CI/VASEx-CI/cds-libs:gcc-6-cxxflags:       -march=native -std=c++14
build/CI/VASEx-CI/cds-libs:gcc-7-cxxflags:       -march=native -std=c++1z
build/CI/VASEx-CI/cds-libs:gcc-8-cxxflags:       -march=native  -std=c++17 -Wmultistatement-macros
build/CI/VASEx-CI/cds-libs:clang-4-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++14
build/CI/VASEx-CI/cds-libs:clang-5-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++1z
build/CI/VASEx-CI/cds-libs:clang-6-cxxflags:       -stdlib=libc++ -Wdocumentation -std=c++17