AC_PROG_LEX issue when compiling with -Werror

[Tobias Brunner <tobias@xxxxxxxxxxxxxx>]

Hi all,

On FreeBSD, the Autoconf port was recently updated to 2.71.  This had 
the side-effect that our automated CI builds failed on that platform 
because our lexers were not generated anymore.

As it turned out, the reason is that we build with -Werror in CFLAGS 
(passed directly to the configure script) and that, in turn, caused 
AC_PROG_LEX to fail the detection of the lex library (which we don't 
actually need as our lexers set `%option noyywrap` and I passed noyywrap 
also to the AC_PROG_LEX macro - still, the script looks for a library). 
This wasn't an issue with Autoconf 2.69 that was used before.

When building without -Werror I see this:

  checking for flex... flex
  checking for lex output file root... lex.yy
  checking for lex library... none needed

With -Werror included in CFLAGS the result is this:

  checking for flex... flex
  checking for lex output file root... lex.yy
  checking for lex library... not found
  configure: WARNING: required lex library not found; giving up on flex

This is caused by the following warnings turned errors from the 
generated lexer:

lex.yy.c:673:13: error: misleading indentation; statement is not part of 
the previous 'if' [-Werror,-Wmisleading-indentation]
            if ( ! (yy_state_buf) )
            ^
lex.yy.c:671:9: note: previous statement is here
        if ( ! (yy_state_buf) )
        ^
lex.yy.c:1107:3: error: misleading indentation; statement is not part of 
the previous 'if' [-Werror,-Wmisleading-indentation]
                return yy_is_jam ? 0 : yy_current_state;
                ^
lex.yy.c:1104:2: note: previous statement is here
        if ( ! yy_is_jam )
        ^

Note that these warnings are not triggered for our own lexers, 
presumably because we don't use REJECT, which seems to generate the 
above code.  That is, building with -Werror works just fine if the 
library check is bypassed.

So I wonder how we should workaround this.  In particular, if anybody 
has any recommendations on how to handle -Werror when we explicitly want 
to fail our automated builds based on warnings without affecting the 
tests in configure.  Is there a proper way to do this with Autoconf? 
Should we e.g. add a --enable-werror option (that modifies CFLAGS late 
in the script)?  Or provide something like EXTRA_CFLAGS (which would 
again get added to CFLAGS late in the script)?

Thanks,
Tobias