An extension target is a
make target that you write to accomplish some build goal that Automake doesn't automatically support. A recursive extension target is one that traverses your project directory structure, visiting every Makefile.am file in your Autotools build system and giving each one the opportunity to do some work when the extension target is made.
When you add a new top-level target to your build system, you have to either tie it into an existing Automake target, or add your own
make code to the desired target that traverses the subdirectory structure provided by Automake in your build system.
SUBDIRS variable is used to recursively traverse all subdirectories of the current directory, ...