CMakePM: Do not filter out all UTILITY targets

add_custom_target will be set as UTILITY target, which one might
want to reference as "cm <target>" or "--target <target>".

Filter out instead the "_autogen" and "_autogen_timestamp_deps".

Fixes: QTCREATORBUG-25726
Change-Id: Ia73d6e87e4b230114d068649964792ff3f626378
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsystem.h

@@ -99,6 +99,8 @@ public:
     static CMakeConfig parseCMakeCacheDotTxt(const Utils::FilePath &cacheFile,
                                              QString *errorMessage);
 
+    static bool filteredOutTarget(const CMakeBuildTarget &target);
+
     bool isMultiConfig() const;
     bool usesAllCapsTargets() const;