Fix the build on FreeBSD versions with libc++ as default.
The failure mode looked as follows:
In file included from /scratch/tmp/gerald/gcc-HEAD/libcpp/system.h:442: In file included from /usr/include/c++/v1/new:91: /usr/include/c++/v1/exception:180:5: error: no member named 'fancy_abort' in namespace 'std::__1'; did you mean simply 'fancy_abort'? _VSTD::abort(); ^~~~~~~