Explain custom configuration files.

Avoid Nonetype-related errors.

TODO: Would be better to sanitize every parameter at beginning.

because the output seems broken with enabling thread safe.

(librsvg 2.37 requires newer pango.)

* enable thread safe
* build without libidn

Tests directory of inserted paths also needs to be in sys.path so that tests
can import other tests.

our specified revision seems to be disappeared maybe by force push.

Old versions of slapproxy/slapos master don't know it.
Catching it here so that we have a chance to debug problems in SRs.