[octave:config] Build errors on default

[Juan Pablo Carbajal]

In ubuntu 17.10 I get the following error when compiling default

  CXX      libinterp/operators/libinterp_liboctinterp_la-op-bm-b.lo
../default/libinterp/version.cc: In function ‘std::__cxx11::string
octave_name_version_copyright_copying_and_warranty(bool, const
string&)’:
../default/libinterp/version.cc:92:20: error: ‘octave::config’ has not
been declared
          + octave::config::canonical_host_type ()
                    ^~~~~~
Makefile:18770: recipe for target
'libinterp/libinterp_liboctinterp_la-version.lo' failed

Declaration and definition of the function in defaults.{h,cc} seems
ok. Maybe the object is not linked?