10.5 Defining and Redefining Pattern Rules
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 section How to Use Variables, and Functions for Transforming Text.