Re: Possible bug: AC_USE_SYSTEM_EXTENSIONS ignores "AS_IF" conditionals

[Paul Eggert]

On 09/22/2014 09:07 AM, David A. Wheeler wrote:
> I want to write code that will*use*  extensions where available, but will 
> portably run on
> systems that don't have the extensions.

That sounds like a reasonable thing to do, but unfortunately autoconf 
doesn't support it directly.  The best workaround I can come up with off 
the top of my head, is to copy the definition of 
AC_USE_SYSTEM_EXTENSIONS from the Autoconf source, rename the copy to 
dw_USE_SYSTEM_EXTENSIONS, and define the copy with AC_DEFUN rather than 
AC_DEFUN_ONCE.  It might be helpful to extend Autoconf to provide a 
non-ONCE version of AC_USE_SYSTEM_EXTENSIONS, so that this kind of 
hackery wasn't needed.