[RTEMS Project] #3311: Update waf.py for Sphinx 1.7 compatibility

[RTEMS trac]

#3311: Update waf.py for Sphinx 1.7 compatibility
-------------------------------+--------------------
  Reporter:  Richard Campbell  |      Owner:  (none)
      Type:  defect            |     Status:  new
  Priority:  normal            |  Milestone:
 Component:  doc               |    Version:
  Severity:  normal            |   Keywords:
Blocked By:                    |   Blocking:
-------------------------------+--------------------
 sphinx-build --version in sphinx 1.7 seems to return its version
 information in stderr instead of stdout.

 Added try/except blocks to the check_sphinx_version method to attempt to
 get version from sphinx.version_info then from cmd_and_log using the
 output-Context.BOTH parameter and finally from cmd_and_log with no
 additional argument.

 Deleted unused import from sphinx.util.compat in rtemsdomain.py. This
 module was removed in sphinx 1.7.

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Ticket URL: <http://devel.rtems.org/ticket/3311>
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