* Better cxx11/14 flags for GNU/Clang/Intel
- GCC 4.8 only supports -std=c++1y for C++14
- Use CMake's rules for AppleClang to set cxx11 and cxx14 flags based on
Apple Xcode/LLVM version
- Use CMake's rules for Intel to add support for cxx14 flags based on
Intel version.
* Add cxx17_flag property
Implement property in compiler for c++17 as per those for c++11/14.
Add concrete support for GNU/Clang:
- Return -std=c++1z for GCC 5 and above per GCC documentation
- Return -std=c++1z for Clang 3.5 and above per Clang documentation
- Return -std=c++1z for Apple LLVM 6.1 and above per CMake's rules
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