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

[Vincent Lefevre]

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html
says about the target rules:

  .POSIX
    The application shall ensure that this special target is specified
    without prerequisites or commands. If it appears as the first
    non-comment line in the makefile, /make/ shall process the makefile
    as specified by this section; otherwise, the behavior of /make/ is
    unspecified.

But even though one may add a .POSIX target as the first non-comment
line in one's Makefile.am file, Automake will add various non-comment
lines before this target in the generated Makefile. I received a
remark about that for GNU MPFR. Though GNU make does not require
this target to be the first non-comment line, this may matter with
other make implementations.

This could be done either by detecting a .POSIX target in Makefile.am
or with some AM_* macro in the configure.ac file.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)