``cable_add_cxx_compiler_flag_if_supported`` breaks options that rely on slashes being preserved

@axic suggested I mention this here: in the solidity repo we have to deviate from cable's ``cable_add_cxx_compiler_flag_if_supported``, due to the regex in
https://github.com/ethereum/cable/blob/ccd2732e8c11d33cddb6b43ce4e9850dc70fbbac/CableCompilerSettings.cmake#L34

Specifically we have ``eth_add_cxx_compiler_flag_if_supported("-fmacro-prefix-map=${CMAKE_SOURCE_DIR}=/solidity")`` in
https://github.com/ethereum/solidity/blob/0015e8cd847ae81926438e7f8aa90b89ed328e0c/cmake/EthCompilerSettings.cmake#L29
which relies on slashes that are not at the beginning of the option to be left untouched.

We noticed when @axic attempted to re-sync our ancient ``eth_add_cxx_compiler_flag_if_supported`` with the current ``cable_add_cxx_compiler_flag_if_supported`` from here.

Start in CableCompilerSettings.cmake at the regex around line 34 and compare it with the Solidity usage in cmake/EthCompilerSettings.cmake at line 29. Check how cable_add_cxx_compiler_flag_if_supported handles -fmacro-prefix-map=${CMAKE_SOURCE_DIR}=/solidity, then verify that slashes not at the beginning of the option remain unchanged and that Cable no longer needs the Solidity-specific deviation.

Written by the indexing model from the issue text.