Question

What is EXTRA_SCRIPTS variable for in Makefile.am? Where is it documented?

Answer 1

From the automake manual:

For each primary, there is one additional variable named by prepending ‘EXTRA_’ to the primary name. This variable is used to list objects that may or may not be built, depending on what configure decides. This variable is required because Automake must statically know the entire list of objects that may be built in order to generate a Makefile.in that will work in all cases.

The SCRIPTS primary is described here.

In short, the EXTRA_SCRIPTS variable lists scripts (that is, executable files that do not need to be built from source) that may or may not be used, depending on the configuration.