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(autoconf.info)Header Templates

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Configuration Header Templates
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   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