Support externally built RIME plugins

[Arfrever]

For packaging of RIME plugins, it is preferred to allow to build and install them separately.

• This patchset does not remove pre-existent support for building "plugins" in librime source tree and does not change default behavior.

• Externally built plugins should be installed to separate directory. By analogy with ${prefix}/share/rime-data, ${prefix}/${libdir}/rime-plugins is used by default (e.g. /usr/lib/rime-plugins or /usr/lib64/rime-plugins) and can be changed by setting RIME_PLUGINS_DIR variable during invocation of cmake.

E.g. librime-lua, with simple change to support this solution, would install e.g. /usr/lib64/rime-plugins/lua.so file.

[Arfrever]

Patch: librime-externally_built_plugins.patch.txt

Patch: librime-externally_built_plugins.patch.txt

[lotem]

A status update: I'm still working on a few changes to the patch.

One issue is the mismatch between plugin file name and Rime module name. I have a solution for that already.
Also the calling site in the patch code isn't perfect as it will also load the plugins unnecessarily when a deployer process, instead of the IME service proper, is running.
The idea is to refactor the plugin.so loading logic into a plugins module, which is optionally appended to the RIME_EXTRA_MODULES array.
I'll send a pull request when more ready.

[lotem]

Merged as b2abd09