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.
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 `header-to-create.in'; however, you can override the input file name by appending to header-to-create, a colon-separated list of input files. Examples:
AC_CONFIG_HEADER(defines.h:defines.hin) AC_CONFIG_HEADER(defines.h:defs.pre:defines.h.in:defs.post)
Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.
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