ENH: Use std::once_flag for GlobalSingletonIndexInitializer
ENH: Use std::once_flag for GlobalSingletonIndexInitializer

richardbeare/ITK,hjmjohnson/ITK,thewtex/ITK,BRAINSia/ITK,richardbeare/ITK,hjmjohnson/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,hjmjohnson/ITK,BRAINSia/ITK,richardbeare/ITK,Kitware/ITK,thewtex/ITK,BRAINSia/ITK,richardbeare/ITK,Kitware/ITK,thewtex/ITK,Kitware/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,thewtex/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,richardbeare/ITK,Kitware/ITK,hjmjohnson/ITK,BRAINSia/ITK,BRAINSia/ITK,thewtex/ITK,Kitware/ITK,richardbeare/ITK