configure: test for group xenomai, and warn if user not in group xenomai
configure: test for group xenomai, and warn if user not in group xenomai

this really is for xenomai-user only:

group xenomai would typically indicate an installation problem or
missing xenomai userland support

a user must be member of group xenomai to call upon xenomai RT functions
we warn here, but dont fail, because it could well be the build user and
the user executing linuxcnc are not the same

cdsteinkuehler/linuxcnc,bobvanderlinden/machinekit,cdsteinkuehler/linuxcnc,cdsteinkuehler/linuxcnc,mhaberler/machinekit,mhaberler/machinekit,EqAfrica/machinekit,Cid427/machinekit,bobvanderlinden/machinekit,RunningLight/machinekit,Cid427/machinekit,unseenlaser/machinekit,ArcEye/MK-Qt5,Cid427/machinekit,Cid427/machinekit,EqAfrica/machinekit,cdsteinkuehler/linuxcnc,cdsteinkuehler/MachineKit,ArcEye/MK-Qt5,EqAfrica/machinekit,strahlex/machinekit,mhaberler/machinekit,cdsteinkuehler/linuxcnc,araisrobo/machinekit,cdsteinkuehler/MachineKit,araisrobo/machinekit,cdsteinkuehler/linuxcnc,unseenlaser/machinekit,Cid427/machinekit,ArcEye/MK-Qt5,araisrobo/machinekit,mhaberler/machinekit,EqAfrica/machinekit,bobvanderlinden/machinekit,bobvanderlinden/machinekit,RunningLight/machinekit,RunningLight/machinekit,strahlex/machinekit,araisrobo/machinekit,ArcEye/machinekit-testing,bobvanderlinden/machinekit,unseenlaser/machinekit,kinsamanka/machinekit,ArcEye/MK-Qt5,araisrobo/machinekit,RunningLight/machinekit,kinsamanka/machinekit,bobvanderlinden/machinekit,RunningLight/machinekit,kinsamanka/machinekit,kinsamanka/machinekit,unseenlaser/machinekit,ArcEye/machinekit-testing,Cid427/machinekit,araisrobo/machinekit,unseenlaser/machinekit,unseenlaser/machinekit,mhaberler/machinekit,ArcEye/machinekit-testing,EqAfrica/machinekit,unseenlaser/machinekit,Cid427/machinekit,cdsteinkuehler/MachineKit,strahlex/machinekit,strahlex/machinekit,Cid427/machinekit,ArcEye/MK-Qt5,araisrobo/machinekit,unseenlaser/machinekit,ArcEye/machinekit-testing,bobvanderlinden/machinekit,EqAfrica/machinekit,ArcEye/MK-Qt5,kinsamanka/machinekit,araisrobo/machinekit,RunningLight/machinekit,mhaberler/machinekit,mhaberler/machinekit,ArcEye/machinekit-testing,EqAfrica/machinekit,kinsamanka/machinekit,araisrobo/machinekit,RunningLight/machinekit,cdsteinkuehler/MachineKit,EqAfrica/machinekit,strahlex/machinekit,mhaberler/machinekit,cdsteinkuehler/MachineKit,ArcEye/MK-Qt5,RunningLight/machinekit,strahlex/machinekit,ArcEye/machinekit-testing,kinsamanka/machinekit,cdsteinkuehler/MachineKit,ArcEye/machinekit-testing,ArcEye/machinekit-testing,bobvanderlinden/machinekit,ArcEye/MK-Qt5,kinsamanka/machinekit,strahlex/machinekit