RFC: Changing AC_DEFINE

[Akim Demaille]

I would like to address the issues raised in
http://gcc.gnu.org/ml/gcc/2001-03/msg00814.html for 2.51.  This
basically means try to use `echo' instead of a heredoc when there are
no funky characters.

Another idea that _might_ improve performances too would be to use a
fd for confdefs.h, just like for config.log.  This raises two
questions:

1. what is the next fd we can use?  Is 7 OK?

2. is it safe to use both an FD to confdefs.h and redirections to
confdefs.h?


Just for the record, the figures are quite impressive:

   |       Subject: Re: libstdc++ configure time 
   |       From: Richard Henderson <rth at redhat dot com> 
   | 
   | I've hacked autoconf to use echo in a number of key places.
   | 
   | With just AC_DEFINE, AC_DEFINE_UNQUOTED, and AC_TRY_LINK,
   | the number of here documents is down from 485 to 156.  This
   | takes 16:20.3 to run configure on Solaris.
   | 
   | Transforming AC_TRY_COMPILE, AC_TRY_CPP bring the number
   | down to 29.  A few more here and there bring us to 21.
   | This takes 2:14.7 to run configure.
   | 
   | 
   | r~


Note: I will _not_ use the solution they advocate (e.g., in
http://gcc.gnu.org/ml/gcc/2001-03/msg00975.html), but something based
on AC_VAR_IFELSE or so which will decide, case by case, whether echo
can be used, or whether heredocs should.