Search notes:

configure.ac

autoscan might help in creating a configure.ac.
Based on the content of the configure.ac file, /usr/share/autoconf (or autoreconf) creates the ./configure script.

Shell script

configure.ac is a shell script with macro instructions.
So, it would be possible to include common shell script inststructions. However, it is not uncommon to find configure.ac files that consist of macro instructions only.

Standard configure.ac layout

Only two macros are required in a configure.ac file: AC_INIT and AC_OUTPUT. 
AC_PREREQ(version)
AC_INIT(package, version, bug-report-address)

package info
checks:
  - programs
  - libraries
  - header files
  - types
  - structures
  - compiler characteristics
  - library functions
  - system services

AC_CONFIG_FILES([file …])
AC_OUTPUT

configure.in

Earlier versions of Autoconf produced configure.in.
The preferred name is configure.ac.

See also

configure.in
autoreconf

Index