[CMake] Default CMAKE_C_FLAGS value by OS

[Chuck Atkins]

Hi YC,


> cmake version on fedora 25: 3.6.2
>

Fedora is specifically patching CMake in the RPM spec file to reduce the
gcc flag from O3 to O2 so it's the distributuion's packaging making that
change, not CMake itself.  If you download and build the source from
cmake.org then you'll get O3 as default for gcc release builds.

That being said, I do think we should revisit the use of O3 by default
given the safety issues surounding it.

- Chuck
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