[m-rev.] for review: Require readline/editline header files to use the libraries.

[Julien Fischer]

On Tue, 4 Jun 2019, Peter Wang wrote:

Add a summary line (e.g. the subject of this mail) to the start of the
log message; it looks strange in git log otherwise

> m4/mercury.m4:
>    Require readline headers in order to use the readline library.
>
>    Require editline header in order to use the editline library.


What's the rationale for this change?

The diff is fine.

Julien.