[m-rev.] for review: Make gcc and clang version checks more robust.

[Julien Fischer]

On Tue, 19 Dec 2023, Peter Wang wrote:

> {x86_64,i686}-w64-mingw32-gcc -dumpfullversion on Debian 12
> reports "12-win32", which the MERCURY_GCC_VERSION macro failed
> to parse, as it expected a minor version to be present.

Really? Whose idea was that?!? That isn't the documented behaviour
of that option.

> m4/mercury.m4:
>    In the MERCURY_GCC_VERSION macro:
>      - simplify calling $CC -dumpfullversion or $CC -dumpversion
>      - trim any suffix following a hyphen in the raw gcc version output
>      - set major/minor/patchlevel fields containing anything
>        other than a sequence of decimal digits to "u"
>      - use $() syntax for command substitutions
>
>    Modify the MERCURY_CLANG_VERSION macro in a similar way.

That's fine.

Julien.