ProjectExplorer: Use a functor for BuildSystem creation

... instead of creating the BuildSystem direct. This will help the
shift of BuildSystem owner ship as a Project will have potentially
multiple BuildSystem instances (one per BuildConfiguration), but
still be responsible for creating them with the Targets.

Change-Id: I2dd71c7687ed41af9e42c874b3f932ce704e7ee3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -91,7 +91,7 @@ CMakeProject::CMakeProject(const FilePath &fileName)
     setKnowsAllBuildExecutables(false);
     setHasMakeInstallEquivalent(true);
 
-    setBuildSystem(std::make_unique<CMakeBuildSystem>(this));
+    setBuildSystemCreator([](Project *p) { return new CMakeBuildSystem(p); });
 }
 
 CMakeProject::~CMakeProject() = default;