CMake: Select better default target when a buildstep's target is no longer found

Select a better fallback target when a buildstep's target is no longer defined.

This change was cherry-picked from master branch:
9519f125e9

Change-Id: I7ef27361793125258cd2a73448d91d2561db5901
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.cpp

@@ -115,7 +115,7 @@ void CMakeBuildStep::handleBuildTargetChanges(bool success)
         return; // Do not change when parsing failed.
     if (!isCurrentExecutableTarget(m_buildTarget)
         && !static_cast<CMakeProject *>(project())->buildTargetTitles().contains(m_buildTarget)) {
-        setBuildTarget(allTarget());
+        setBuildTarget(defaultBuildTarget());
     }
     emit buildTargetsChanged();
 }