Add XKB_CONFIG_ROOT environment

[mvo5]

Trivial patch that allows overriding the DFLT_XKB_CONFIG_ROOT path via a new XKB_CONFIG_ROOT environment to make it possible to set the path at runtime instead of build-time (to make binaries build with libxkbcommon easier relocatable).

[bluetech]

I'm not a big fan of environment variables, but we have precedent, and this seems logical, so I'm OK with it. Just two comments:

1. Please use secure_getenv.
2. Please add a note about the effect of this envvar. Hmm actually, the docs could use some expanding about the default include path anyway, so you can leave this to me.

Thanks.

[mvo5]

Thanks a lot for you quick reply! I updated the code to use secure_getenv() and also moved the *root declaration up next to *home to match the existing code better.

I'm also happy to help with the documentation for the new environment, I poked a bit around and could not quite find a place to start so I'm happy to leave it to you. But please let me know if I can help in any way.

[bluetech]

Thanks!

I'll add a section to the documentation about the include path handling soon, and include a note about XKB_CONFIG_ROOT.