Fix unknown c++ flag detection in CMake

[cyyever]

Unknown -Wno-XXX flags are still appended to GCC via append_cxx_flag_if_supported because of the behavior mentioned in GCC document:

When an unrecognized warning option is requested (e.g., -Wunknown-warning),
GCC emits a diagnostic stating that the option is not recognized.
However, if the -Wno- form is used, the behavior is slightly different:
no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced.
This allows the use of new -Wno- options with old compilers,
but if something goes wrong, the compiler warns that an unrecognized option is present.

This PR tries to fix by detection the flag of the -WXXX form. The same patch was applied in PyTorch.

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@q10 merged this pull request in 17384a9.