Why are SDL macros defined in acinclude.m4?

I noticed that SDL’s acinclude.m4 includes stuff
like this:

Local macros for the SDL configure.in script

dnl Function to copy an architecture specific file
dnl COPY_ARCH_SRC(source_dir, arch, source_file)

— libtool.m4 —#

libtool.m4 - Configure libtool for the target system. --Shell-script--

Copyright © 1996-1999 Free Software Foundation, Inc.

Originally by Gordon Matzigkeit , 1996

I can understand why the libtool macros are there, but why
are SDL macros also in there? Why not put those in sdl.m4?
That would make it easier to copy a new libtool.m4 file
over acinclude.m4.

Mo DeJong
Red Hat Inc