Define tunnel variables for bypassing fcitx limitations

[ksqsf]

Pass UI settings directly from f5m to librime. The approach here is to define some variables in tunnel.cpp, and the dynlib loader will automatically resolve undefined symbols to the definitions in the main program. However, we have to pass some linker flags to silence linker complaints. I don't expect the mechanism to be used in the future, except for hacks like this.

How to test

1. Build and install the new f5m main program
2. Build and install the new librime (rime: update selector to respect f5m settings fcitx5-macos-prebuilder#44)
3. Build and install the new fcitx5-rime (rime: respect f5m UI settings fcitx5-macos-plugins#48)
4. Launch the new f5m

After this change, the new f5-rime plugin (shipped by f5m) will require the latest main program to function, or it won't be loaded (with an error message similar to 'undefined symbol' in Fcitx5.log).

Please merge prebuilder first and then merge the plugin to make sure the plugin is linked with the patched librime.

[eagleoflqj]

Tested 6 combinations with rime. LGTM!