`-fcf-protection=check` is not currently supported by Clang

[ckwastra]

Reference:

wg-best-practices-os-developers/docs/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.md

Line 760 in b14d7a9

| <span id="-fcf-protection=check">`-fcf-protection=check`</span><br/> | GCC 8.0.0<br/>Clang 7.0.0 | Instruct linker to verify all object files in final link with link-time optimization (LTO) are compiled with identical control-flow protections |

Passing the -fcf-protection=check flag to Clang results in the following error:

error: invalid value 'check' in '-fcf-protection=check'

[thomasnyman]

Thanks, you are indeed correct. The check argument appears to be GCC-specific. Clang supports only return, branch, full or none. This is fixed in #916.