Configuration Headers

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Configuration Header Files

When a package contains more than a few tests that define 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_HEADERS' 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.
     This macro is one of the instantiating macros; see *Note
     Configuration Actions::.  Make `AC_OUTPUT' create the file(s) in
     the whitespace-separated list HEADER 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 is `config.h'.
     If HEADER already exists and its contents are identical to what
     `AC_OUTPUT' would put in it, it is left alone.  Doing this allows
     making some changes in the 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 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.
   *Note Configuration Actions::, for more details on HEADER.