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

[Vincent Lefevre]

On 2023-01-14 01:30:24 -0500, Nick Bowler wrote:
> Consider these two Makefiles, identical except for the presence of
> the .POSIX special target:
> 
>   % cat >noposix.mk <<'EOF'
>   craziness:
>       @-false; echo hello
>       @false; echo hello
> EOF
> 
>   % cat >posix.mk <<'EOF'
>   .POSIX:
>   craziness:
>       @-false; echo hello
>       @false; echo hello
> EOF
[...]
> HP-UX 11:
> 
>   (this is the actual POSIX-specified behaviour which I also observed on
>   multiple other systems):
> 
>   % make -f noposix.mk
>   hello
>   *** Error exit code 1
> 
>   Stop.
> 
>   % make -f posix.mk
>   hello
>   *** Error exit code 1

Same behavior on AIX, i.e. the .POSIX special target is useless for
*this* example. However, if it is not the first non-comment line, I
get a warning:

  warning: .POSIX directive is not first non-comment line.

(found during my test of GNU MPFR on cfarm119.cfarm.net).

Note that its "make" man page says:

  .POSIX
      Causes the make command to use a different default rules file. The
      file, /usr/ccs/lib/posix.mk, provides the default rules as
      specified in the POSIX standard.

but I don't know whether this changes the behavior with Automake.

If /usr/ccs/lib/posix.mk is included when the .POSIX directive is read,
the fact that it is not the first non-comment line might cause issues.
For instance, this file (re)defines some variables:

MAKE=make
AR=ar
ARFLAGS=-rv
YACC=yacc
YFLAGS=
LEX=lex
LFLAGS=
LDFLAGS=
CC=c89
CFLAGS=-O
FC=fort77
FFLAGS=-O 1
GET=get
GFLAGS=

So this is expected to be found early in the makefile.

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