Fix unknown c++ flag detection in CMake (#109000)
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. Unfortunately, third_party/fbgemm/CMakeLists.txt redefines append_cxx_flag_if_supported and our version is overwritten. As a result, we have to re-include utils.cmake to overwrite it again.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109000
Approved by: https://github.com/malfet
