When building a project relying on External projects, the launchers were
not used in subprojects built without testing enabled. This was preventing
errors and warnings associated with these subprojects from being
reported on the dashboard.
This commit allows enabling the launchers independently of the value of
"BUILD_TESTING" using one of these two approaches:
1) By setting both CTEST_USE_LAUNCHERS and the env variable
"CTEST_USE_LAUNCHERS_DEFAULT" to 1 in the ctest dashboard driver scripts.
2) By enabling the variable CTEST_USE_LAUNCHERS in the ctest dashboard
driver script and also by ensuring every external project passes the option
-DCMAKE_PROJECT_<projectname>_INCLUDE:FILEPATH=${CMAKE_ROOT}/Modules/CTestUseLaunchers.cmake
