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 3.4.1 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
 

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