introduce buildQbsProjectManager property

this will centralize some conditionals further down the line

Change-Id: I53b26cc29ac3fa098a583d6ac4d28924e5a1f2a3
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>

src/plugins/qbsprojectmanager/qbsprojectmanager.qbs

@@ -26,7 +26,7 @@ QtcPlugin {
         return libs
     }
 
-    condition: project.useExternalQbs || project.qbsSubModuleExists
+    condition: project.buildQbsProjectManager
 
     Depends { name: "Qt"; submodules: [ "widgets", "script" ] }
     Depends { name: "ProjectExplorer" }

src/src.qbs

@@ -15,6 +15,7 @@ Project {
     property path qbs_build_dir: qbs.getenv("QBS_BUILD_DIR")
     property path qbs_source_dir: qbs.getenv("QBS_SOURCE_DIR")
     property bool useExternalQbs: qbs_build_dir && qbs_source_dir
+    property bool buildQbsProjectManager: useExternalQbs || qbsSubModuleExists
     Project {
         name: "qbs"
         id: qbsProject