encapsulating --enable-gcc-warnings [Re: [PATCH] build: work with new glibc when not optimizing

[Jim Meyering]

[cc'ing bug-gnulib]
This started with a useful coreutils patch by Eric,

    http://thread.gmane.org/gmane.comp.gnu.coreutils.general/3164

The thing is, it belongs in nearly every project that
also provides the configure-time --enable-gcc-warnings option:

Mike Frysinger wrote:
> On Sat, Sep 1, 2012 at 12:10 PM, Eric Blake wrote:
>> Starting with glibc 2.15, the system headers refuse to compile
>> unconditional use of FORTIFY_SOURCE if optimization is disabled
>> but -Werror is in effect.
>
> pretty sure you mean 2.16.0
>
>> Pushing, as this is similar to the recent coreutils patch.
>
> too little code for a gnulib module ?  and i guess GNU projects don't
> want to rely on the autoconf-archive (AX_*) ?

I thought the same thing, and wanted to make it the default, somehow,
i.e., piggy-back on another nearly-always-used gnulib module.
However, that added code is in a conditional:

    AC_ARG_ENABLE([gcc-warnings],
      [AS_HELP_STRING([--enable-gcc-warnings],
                      [turn on lots of GCC warnings (for developers)])],
      [case $enableval in
         yes|no) ;;
         *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
       esac
       gl_gcc_warnings=$enableval],
      [gl_gcc_warnings=no]
    )

    if test "$gl_gcc_warnings" = yes; then
      gl_WARN_ADD([-Werror], [WERROR_CFLAGS])
      AC_SUBST([WERROR_CFLAGS])

      nw=
      # This, $nw, is the list of warnings we disable.
      nw="$nw -Wdeclaration-after-statement" # too useful to forbid
      nw="$nw -Waggregate-return"       # anachronistic
      nw="$nw -Wlong-long"              # C90 is anachronistic 
(lib/gethrxtime.h)
      nw="$nw -Wc++-compat"             # We don't care about C++ compilers
      nw="$nw -Wundef"                  # Warns on '#if GNULIB_FOO' etc in 
gnulib
      nw="$nw -Wtraditional"            # Warns on #elif which we use often
      nw="$nw -Wcast-qual"              # Too many warnings for now
      ...

and the list of -W options we disable varies from project to project.
On the other hand, many projects have a block very similar to this one,
so one possibility is to factor out this configure-time
--enable-gcc-warnings option and its code, including Eric's latest addition.

If someone wants to create a new module that is configurable enough
to be used by these packages (the ones I have handy, and for which
"grep -l enable-gcc-warning" declared a match):

    bison
    coreutils
    cppi
    diffutils
    emacs
    gnutls
    grep
    gsasl
    gss
    gzip
    hivex
    idutils
    iwhd
    libguestfs
    parted
    patch
    shishi
    zile

that would be some serious factoring.  In coreutils, that code
occupies more than 100 lines of configure.ac.

The more we can move out of individual package configure.ac files
and into gnulib, where we can centralize improvements, the better.