Homebrew build fixes

[ret2libc]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

• allow packager to use absolute paths for plugins/wwwroot/sdb/etc. directories, even outside the prefix

Test plan

• compile regularly
• compile with -Drizin_plugins=/system/dir/outside/prefix

Closing issues

...

[XVilka]

Maybe create a CI job that tests some simple plugin? Like we test pkg-config/cmake availability on Windows, for example?