23.1 Autoconf Preliminaries
In a small package which only uses Autoconf, your own macros are placed
in the `aclocal.m4' file--this includes macros that you may have
obtained from third parties such as the Autoconf macro archive
(see section 23.5.1 Autoconf macro archive). If your package additionally uses
Automake, then these macros should be placed in `acinclude.m4'.
In larger projects, it's advisable to keep your custom macros in a more organized structure. Autoconf version 2.15 will introduce a new facility to explicitly include files from your `configure.in' file. The details have not solidified yet, but it will almost certainly include a mechanism for automatically included files with the correct filename extension from a subdirectory, say `m4/'.