cmake: Set macOS target for blst only if understood

[chfast]

In some configurations (e.g. Silkworm / conan) the CMAKE_OSX_DEPLOYMENT_TARGET may be defined even if we don't target macOS. Pass this value to the C compiler only if it supports the deployment target flag (-mmacos-version-min=).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.19%. Comparing base (7961b60) to head (6fcf3c5). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1028 +/- ## ======================================= Coverage 94.19% 94.19% ======================================= Files 149 149 Lines 15888 15888 ======================================= Hits 14965 14965 Misses 923 923 ``` | [Flag](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | Coverage Δ | | |---|---|---| | [eof_execution_spec_tests](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | `17.25% <ø> (ø)` | | | [ethereum_tests](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | `27.33% <ø> (ø)` | | | [ethereum_tests_silkpre](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | `19.13% <ø> (ø)` | | | [execution_spec_tests](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | `20.42% <ø> (ø)` | | | [unittests](https://app.codecov.io/gh/ethereum/evmone/pull/1028/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum) | `88.96% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ethereum#carryforward-flags-in-the-pull-request-comment) to find out more.