Libtool comes with a tool called libtoolize that will
install libtool’s supporting files into a package. Running this
command will install ltmain.sh. You should execute it before
aclocal and automake.
People upgrading old packages to newer autotools are likely to face
this issue because older Automake versions used to call
libtoolize. Therefore old build scripts do not call
libtoolize.
Since Automake 1.6, it has been decided that running
libtoolize was none of Automake’s business. Instead, that
functionality has been moved into the autoreconf command
(see autoreconf Invocation).
If you do not want to remember what to run and
when, just learn the autoreconf command. Hopefully,
replacing existing bootstrap.sh or autogen.sh scripts by
a call to autoreconf should also free you from any similar
incompatible change in the future.