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.

[julienmalik]

Hum... quite strange that the mingw warnings disappear with ctest launchers enabled. This should be investigated further.

[julienmalik]

CTEST_USE_LAUNCHERS is even ignored for this mingw build since it uses ctest 2.8, which ignores CTEST_USE_LAUNCHERS since include(CTest) is not used. We need cmake >= 3.0 here

[julienmalik]

With cmake 3.5 it gets better : http://my.cdash.org/viewBuildError.php?type=1&buildid=953977
The warnings are caught and the dashboard report is clear.

[julienmalik]

@mayeut ok for merging ?

might want to still have a cmake 2.8 build around though (I see that appveyor also uses cmake 3.5)...

[mayeut]

@julienmalik,

If I understand correctly, there's no more build using cmake 2.8.x.

find . -name 'CMakeLists.txt' -exec grep 'cmake_minimum_required' {} + outputs the following:

./CMakeLists.txt:cmake_minimum_required(VERSION 2.8.2)
./tests/nonregression/CMakeLists.txt:cmake_minimum_required(VERSION 2.8.7)
./thirdparty/libtiff/CMakeLists.txt:cmake_minimum_required(VERSION 2.6)
./thirdparty/libz/CMakeLists.txt:cmake_minimum_required(VERSION 2.6)

It would be good to keep at least one building with cmake 2.8.x. (e.g. I used a feature from cmake 3.x in a PR and didn't realize it until CI checks failed).
I run tests using cmake 2.8.7 once in a while but not frequently enough to assess anything.