[m-rev.] for post-commit review: Fix configure script for compatibility with old versions of bash.

[Julien Fischer]

On Mon, 8 Jan 2024, Peter Wang wrote:

> m4/mercury.m4:
>    Replace $( ) with backticks in commands that confuse the parser in
>    old versions of bash.
>
>    Use 'read -r' instead of plain 'read' so that backslash is not
>    treated as an escape character, not that we expect gcc or clang
>    to print backslashes in the version output.

That looks fine.

Julien.