[m-rev.] for review: Delete unnecessary test for sa_sigaction field.

[Julien Fischer]

On Tue, 13 Oct 2020, Peter Wang wrote:

> configure.ac:
>    Delete check for sa_sigaction field. If a call to sigaction()
>    has the SA_SIGINFO flag set then the handler must be specified in
>    the sa_sigaction field, not the sa_handler field.
>
> runtime/mercury_conf.h.in:
>    Delete MR_SIGACTION_FIELD macro.
>
> runtime/mercury_signal.c:
>    Don't use MR_SIGACTION_FIELD macro.
>
>    Don't define dummy value for SA_SIGINFO.
>
> tools/configure_mingw_cross:
>    Don't set now-unused variable.

That's fine.

Julien.