If I run the following standard commands on my Ubuntu machine :

git clone  https://github.com/php/php-src.git
./buildconf
./configure

This produces, among other things, a Makefile, which at line 1614 has the following rule :

ext/opcache/opcache.la: $(shared_objects_opcache) $(OPCACHE_SHARED_DEPENDENCIES)

It seems this OPCACHE_SHARED_DEPENDENCIES is nowhere defined inside the makefile. How then does make attribute a value to it ?
What am I missing here ?