autoconf.info: Writing configure.ac

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Writing `configure.ac'

To produce a `configure' script for a software package, create a file
called `configure.ac' that contains invocations of the Autoconf macros
that test the system features your package needs or can use.  Autoconf
macros already exist to check for many features; see *Note Existing
Tests::, for their descriptions.  For most other features, you can use
Autoconf template macros to produce custom checks; see *Note Writing
Tests::, for information about them.  For especially tricky or
specialized features, `configure.ac' might need to contain some
hand-crafted shell commands; see *Note Portable Shell::.  The
`autoscan' program can give you a good start in writing `configure.ac'
(*note autoscan Invocation::, for more information).
   Previous versions of Autoconf promoted the name `configure.in',
which is somewhat ambiguous (the tool needed to process this file is not
described by its extension), and introduces a slight confusion with
`config.h.in' and so on (for which `.in' means "to be processed by
`configure'").  Using `configure.ac' is now preferred.