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();
^~~~~~~