bug#55025: Automake should allow one to enable POSIX make behavior

[Vincent Lefevre]

On 2023-01-14 01:30:24 -0500, Nick Bowler wrote:
> Does adding .POSIX: to a Makefile actually solve any real-world
> portability problem with Automake generated makefiles, or is all of
> this just hypothetical?

For MPFR, when I initially added it, this was just in case.

The real issue at that time was that I was using $< in a case
where it was unspecified by POSIX, and this was failing on FreeBSD.
Unfortunately there doesn't seem to be a way to get a failure
on non-portable features (which would ease maintenance). Though
I test the tarball on various systems before a release candidate,
this is not the case of the development branch (and the Makefiles
would typically be more affected than the C code, on which various
portability checks are done under Linux).

Well, I use autoreconf with --warnings=all,error (passed to automake),
but I didn't get any portability error.

Later I noted:

# The ".POSIX" line is needed in particular for GNU "make", so that
# recipes are invoked as if the shell had been passed the -e flag.

as this is really what we want, both for consistency with other
"make" implementations and to be able to detect errors.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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