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
Existing Tests, for their descriptions. For most other features,
you can use Autoconf template macros to produce custom checks; see
Writing Tests, for information about them. For especially tricky
or specialized features, configure.ac might need to contain some
hand-crafted shell commands; see Portable Shell
Programming. The autoscan program can give you a good start
in writing configure.ac (see Using autoscan to Create configure.ac, 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, while the use of configure.in will cause warnings
from autoconf.