[m-rev.] for review: Replace some run-time configure tests with compile-time tests.

[Julien Fischer]

On Mon, 5 Mar 2018, Peter Wang wrote:

> m4/mercury.m4:
>    Add a macro MERCURY_TRY_STATIC_ASSERT for testing a single
>    compile-time expression.
>
> configure.ac:
>    Replace some run-time tests (using AC_TRY_RUN) with compile-time
>    tests (using MERCURY_TRY_STATIC_ASSERT), allowing said tests to work
>    when cross-compiling.
>
>    Delete code handling mercury_cv_bits_per_word = 16;
>    we do not support it.
>
> tools/configure_mingw_cross:
>    Delete some variables that can now be detected by configure when
>    cross-compiling.

That's fine.

Julien.