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.
This divides cmCTestBuildHandler::GenerateDartBuildOutput into three
methods to generate the header, content, and footer components of
Build.xml files. It will allow the content generation to be replaced
later.