adding formula for glbinding

see https://github.com/hpicgs/glbinding
`needs :cxx11` below `depends_on` and `ENV.cxx11` in the `install` method. That said: it seems odd that compilation succeeded on Lion and Mountain Lion if CXX11 is needed; are you sure it's a hard requirement? Would be good to just try and remove it from the test flags and see if that's enough to fix it.