Autoconf version 2 is mostly backward compatible with version 1.
However, it introduces better ways to do some things, and doesn't
support some of the ugly things in version 1. So, depending on how
sophisticated your `configure.in' files are, you might have to do
some manual work in order to upgrade to version 2. This chapter points
out some problems to watch for when upgrading. Also, perhaps your
configure scripts could benefit from some of the new features in
version 2; the changes are summarized in the file `NEWS' in the
Autoconf distribution.
First, make sure you have GNU m4 version 1.1 or higher installed,
preferably 1.3 or higher. Versions before 1.1 have bugs that prevent
them from working with Autoconf version 2. Versions 1.3 and later are
much faster than earlier versions, because as of version 1.3, GNU
m4 has a more efficient implementation of diversions and can
freeze its internal state in a file that it can read back quickly.
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