4. Writing Rules
A rule appears in the makefile and says when and how to remake
certain files, called the rule's targets (most often only one per rule).
It lists the other files that are the prerequisites of the target, and
commands to use to create or update the target.
The order of rules is not significant, except for determining the
default goal: the target for
make to consider, if you do
not otherwise specify one. The default goal is the target of the first
rule in the first makefile. If the first rule has multiple targets,
only the first target is taken as the default. There are two
exceptions: a target starting with a period is not a default unless it
contains one or more slashes, `/', as well; and, a target that
defines a pattern rule has no effect on the default goal.
(See section Defining and Redefining Pattern Rules.)
Therefore, we usually write the makefile so that the first rule is the
one for compiling the entire program or all the programs described by
the makefile (often with a target called `all').
See section Arguments to Specify the Goals.