Some builds of libffi does not have complex float ffi symbols

  &ffi_type_complex_float,      /*@':csfloat',*/
  &ffi_type_complex_double,     /*@':cdfloat',*/
  &ffi_type_complex_longdouble, /*@':clfloat',*/

in ffi.d refer to libffi symbols. Apparently older versions of libffi (or builds with complex float disabled) do not have these symbols and build errors when ffi.d is being compiled. We should detect this during the call to configure and switch to --with-dffi=included or error when system was explicitly forced.

Issue reported by Dave Richards. Since we've introduced this code recently and it is a regression, it should be fixed before 20.1.x release.