Configuration Header Templates
------------------------------
Your distribution should contain a template file that looks as you
want the final header file to look, including comments, with default
values in the `#define' statements. For example, suppose your
`configure.in' makes these calls:
AC_CONFIG_HEADER(conf.h)
AC_CHECK_HEADERS(unistd.h)
Then you could have code like the following in `conf.h.in'. On systems
that have `unistd.h', `configure' will change the 0 to a 1. On other
systems, it will leave the line unchanged.
/* Define as 1 if you have unistd.h. */
#define HAVE_UNISTD_H 0
Alternately, if your code tests for configuration options using
`#ifdef' instead of `#if', a default value can be to `#undef' the
variable instead of to define it to a value. On systems that have
`unistd.h', `configure' will change the second line to read `#define
HAVE_UNISTD_H 1'. On other systems, it will comment that line out (in
case the system predefines that symbol).
/* Define if you have unistd.h. */
#undef HAVE_UNISTD_H