Buildout output is less scary this way.

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.