[m-rev.] for post-commit review: converting extensions to strings

[Julien Fischer]

On Wed, 21 Jun 2023, Zoltan Somogyi wrote:

> For review by anyone.

The sufficiently recent test in the configure script now fails with:

    Uncaught Mercury exception:
    Software Error: predicate
    `parse_tree.file_names.ext_lib_gs_extension'/2: Unexpected: ext_lib_gs_lib_opt
    Stack dump not available in this grade.

Julien.