-reasons, `configure.in' uses an unholy marriage of custom-baked
-configuration code and autoconf macros. If you want to rebuild
-`configure' from `configure.in', you will need to install a recent
-version of autoconf and GNU m4.
+reasons, `configure.ac' uses an unholy marriage of custom-baked
+configuration code and autoconf macros.
+
+The shell script `autogen.sh' generates 'configure' and other files by
+running the GNU build tools autoconf and automake, which in turn use
+GNU m4 and Perl. If you want to use it, you will need to install
+recent versions of these build tools. This should be needed only if
+you edit files like `configure.ac' that specify Emacs's autobuild
+procedure.