_2 define clashes with libc++
Cloned from https://github.com/ugfem/ug/issues/10
<set> (included in dom/std/std_domain.cc) from libc++ includes
<functional> which defines
placeholders::_2 which clashes with the define
-D_2. This is an issue when compiling UG on e.g. FreeBSD (where I've tested it) and probably also OS X (where I haven't).
A global define _2 was certainly never a good idea. It's not so easy to get rid of now, though. Not only UG but also dune/grid/uggrid.hh would have to be adjusted.