[sr #110346] Revert to looking for yywrap in AC_PROG_LEX

[Zack Weinberg]

URL:
  <https://savannah.gnu.org/support/?110346>

                 Summary: Revert to looking for yywrap in AC_PROG_LEX
                 Project: Autoconf
            Submitted by: zackw
            Submitted on: Fri 30 Oct 2020 09:08:22 PM UTC
                Category: None
                Priority: 5 - Normal
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: None

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Details:

Commit 29ede6b96feee29c0c477d1659081bbdb82cd8b3 caused AC_PROG_LEX to stop
looking for a library that provides yywrap.  This broke several packages in a
Debian archive rebuild:

undefined symbol: yywrap|libmatheval 1.1.11+dfsg-4
undefined symbol: yywrap|snacc 1.3.1-7
undefined symbol: yywrap|splint 1:3.1.2+dfsg-2
undefined symbol: yywrap|tcpxtract 1.0.1-15
undefined symbol: yywrap|wide-dhcpv6 20080615-23

We should go all the way back to the 2.69 behavior, which was to set LEXLIB to
-ll or -lfl if that library defines yywrap, but allow AC_PROG_LEX to succeed
if neither -ll nor -lfl exists on the system, even if a lex program that
doesn't define yywrap would need it.  (This behavior was a bug, but people
have come to depend on it.  See
https://savannah.gnu.org/support/index.php?110269 and the thread starting from
https://lists.gnu.org/r/autoconf-patches/2020-07/msg00013.html for gory
details.)

To provide a path away from bug-compatibility, we could add an optional
argument to AC_PROG_LEX which can be either [yywrap] or [noyywrap], meaning to
look for yywrap and fail configure if it's unavailable, and to not look for
yywrap at all, respectively.




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