( Configuration Headers

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 3.4 Configuration Header Files
 When a package tests more than a few C preprocessor symbols, the command
 lines to pass `-D' options to the compiler can get quite long.  This
 causes two problems.  One is that the `make' output is hard to visually
 scan for errors.  More seriously, the command lines can exceed the
 length limits of some operating systems.  As an alternative to passing
 `-D' options to the compiler, `configure' scripts can create a C header
 file containing `#define' directives.  The `AC_CONFIG_HEADER' macro
 selects this kind of output.  It should be called right after `AC_INIT'.
    The package should `#include' the configuration header file before
 any other header files, to prevent inconsistencies in declarations (for
 example, if it redefines `const').  Use `#include <config.h>' instead
 of `#include "config.h"', and pass the C compiler a `-I.' option (or
 `-I..'; whichever directory contains `config.h').  That way, even if
 the source directory is configured itself (perhaps to make a
 distribution), other build directories can also be configured without
 finding the `config.h' from the source directory.
      Make `AC_OUTPUT' create the file(s) in the whitespace-separated
      list HEADER-TO-CREATE containing C preprocessor `#define'
      statements, and replace `@DEFS@' in generated files with
      `-DHAVE_CONFIG_H' instead of the value of `DEFS'.  The usual name
      for HEADER-TO-CREATE is `config.h'.
      If HEADER-TO-CREATE already exists and its contents are identical
      to what `AC_OUTPUT' would put in it, it is left alone.  Doing this
      allows some changes in configuration without needlessly causing
      object files that depend on the header file to be recompiled.
      Usually the input file is named `'; however,
      you can override the input file name by appending to
      HEADER-TO-CREATE, a colon-separated list of input files.  Examples:
      Doing this allows you to keep your file names acceptable to
      MS-DOS, or to prepend and/or append boilerplate to the file.


* Header Templates            Input for the configuration headers.
* Invoking autoheader         How to create configuration templates.
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