Your distribution should contain a template file that looks as you want
the final header file to look, including comments, with default values
#define statements. For example, suppose your
`configure.in' makes these calls:
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
/* Define if you have unistd.h. */ #undef HAVE_UNISTD_H
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