This defines a 'UseLaunchers' CTest configuration option.  When enabled,
CTest skips log scraping from the Build step output.  Instead it defines
the environment variable CTEST_LAUNCH_LOGS to a log directory during the
build.  After the build it looks for error-*.xml and warning-*.xml files
containing fragments for inclusion in Build.xml and submission.

This is useful in conjuction with 'ctest --launch' and the RULE_LAUNCH_*
properties to get reliable, highly-granular build failure reports.