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Recall that a target that looks like a(m) stands for the member named m in the archive file a.
When make looks for an implicit rule for such a target, as a special
feature it considers implicit rules that match (m), as well as
those that match the actual target a(m).
This causes one special rule whose target is (%) to match. This rule updates the target a(m) by copying the file m into the archive. For example, it will update the archive member target foo.a(bar.o) by copying the file bar.o into the archive foo.a as a member named bar.o.
When this rule is chained with others, the result is very powerful. Thus, `make "foo.a(bar.o)"' (the quotes are needed to protect the `(' and `)' from being interpreted specially by the shell) in the presence of a file bar.c is enough to cause the following commands to be run, even without a makefile:
cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o
Here make has envisioned the file bar.o as an intermediate
file. See Chains of Implicit Rules.
Implicit rules such as this one are written using the automatic variable `$%'. See Automatic Variables.
An archive member name in an archive cannot contain a directory name, but
it may be useful in a makefile to pretend that it does. If you write an
archive member target foo.a(dir/file.o), make will perform
automatic updating with this command:
ar r foo.a dir/file.o
which has the effect of copying the file dir/file.o into a member
named file.o. In connection with such usage, the automatic variables
%D and %F may be useful.