Rules follow the same syntax as rules in Makefiles. See Writing
Rules in GNU
These rules are copied to the output Makefile.in. This allows you to
provide arbitrary functionality for users of the generated Makefiles.
A rule defined in Makefile.am overrides any such
rule of a similar name that would be automatically generated by
automake. Although this is a supported feature, it is not
See also Portable Make Programming.
For instance, the Automake distribution includes a
non-standard rule for the
git-dist target, which the Automake
maintainer uses to make distributions from the source control system.