Allow using CTEST_USE_LAUNCHERS even when tests are disabled

[julienmalik]

Since cmake >= 3.0, using CTEST_USE_LAUNCHERS requires include(CTestUseLaunchers), which is automatically done by include(CTest)

Setting CTEST_USE_LAUNCHERS in the cache without including CTest later triggers a configuration error : CMake Error: CTEST_USE_LAUNCHERS is enabled, but the RULE_LAUNCH_COMPILE global property is not defined. Did you forget to include(CTest) in the toplevel CMakeLists.txt ?

CTEST_USE_LAUNCHERS was silently ignored in cmake <= 2.8 when not including CTest

Include CTestUseLaunchers always, so that CTEST_USE_LAUNCHERS can be used even when not including CTest (for example for MinGW builds). This helps presenting the build errors and warnings more cleanly on the Dashboard.