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/qmakeprojectmanager/qmakeproject.cpp

@@ -145,7 +145,7 @@ QmakeProject::QmakeProject(const FilePath &fileName) :
             this, &QmakeProject::buildFinished);
 
     setPreferredKitPredicate([this](const Kit *kit) -> bool { return matchesKit(kit); });
-    setBuildSystem(std::make_unique<QmakeBuildSystem>(this));
+    setBuildSystemCreator([](Project *p) { return new QmakeBuildSystem(p); });
 }
 
 QmakeProject::~QmakeProject()