You define an implicit rule by writing a pattern rule. A pattern
rule looks like an ordinary rule, except that its target contains the
character % (exactly one of them). The target is considered a
pattern for matching file names; the % can match any nonempty
substring, while other characters match only themselves. The prerequisites
likewise use % to show how their names relate to the target name.
Thus, a pattern rule %.o : %.c says how to make any file
stem.o from another file stem.c.
Note that expansion using % in pattern rules occurs
after any variable or function expansions, which take place
when the makefile is read. See How to Use Variables, and Functions for Transforming Text.