Use GetErrorMode if it is available.

Use GetErrorMode if it is available.

* libltdl/loaders/loadlibrary.c (wrap_geterrormode): New
function that checks if GetErrorMode is supported by the
system and makes use of it if it is.
(fallback_geterrormode): New function that is used otherwise
that implements the old workaround.
(geterrormode): New function pointer that points at either
of the above or directly at GetErrorMode.
(vm_open): Make use of the above.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>