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

[ekpyron]

@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

cable/CableCompilerSettings.cmake

Line 34 in ccd2732

string(REGEX REPLACE "^-|/" "" name ${FLAG})

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.

[axic]

Ping @chfast

[chfast]

I'm not sure where the problem is. The flag is tested as provided, without any modification. The regex replace is only used to have nicer CMake name for the variable (also visible in the output).

If you have a fix for your problem send me a patch.

[chfast]

I remember now that check_cxx_compiler_flag() can fail if "bad" result variable name is provided (for unknown reason).

I created new version if such wrapper in wasmx/fizzy#770. Please review it or even give it a try in solidity.

There I convert flag name to a proper C identifier so it should also fix this issue.