Tim authored Aug 29, 2018 and Brad King committed Aug 30, 2018

Sphinx-build warnings look like this:

    ../path/to/file.rst:105: WARNING: This is not an error.

CTest detects these as errors by this regex:

    ([^ :]+):([0-9]+): ([^ \\t])

Add an exception as we already have for lower-case `: warning`.