RE: Turn on compiler warnings by default for AC_PROG_CC, AC_PROG_CXX & AC_PROG_FC

[Dale Visser]

I've looked over the gnulib warnings module.

> Date: Mon, 10 Feb 2014 16:00:11 -0700
> From: address@hidden
> Ah, but it IS a killer to anyone compiling with -Werror.
> ...
> You really want to borrow some of the ideas from here:
> http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob_plain;f=m4/warnings.m4
>
> gl_COMPILER_OPTION_IF sounds like your proposed AC_APPEND_FLAG_IFVALID,
> with the added benefit of converting -Wno- into -W when probing (gcc
> ...
> Furthermore, that file also has gl_UNKNOWN_WARNINGS_ARE_ERRORS, which
> has various ways of making clang confess which warnings it understands
> ...

All in all, this really looks *much* better than AC_APPEND_FLAG_IFVALID. I will try to substitute in something very close to  gl_WARN_ADD for AC_APPEND_FLAG_IFVALID.

I don't know the numbers, but I suspect gcc and clang together represent a majority of autoconf usage. I will post back when I have tested something I am happy with.