Last-Resort Makefile Targets
You can define a last-resort implicit rule by writing a terminal match-anything pattern rule with no prerequisites (see Match-Anything Rules). This is just like any other pattern rule; the only thing special about it is that it will match any target. So such a rule's recipe is used for all targets and prerequisites that have no recipe of their own and for which no other implicit rule applies.
For example, when testing a makefile, you might not care if the source files contain real data, only that they exist. Then you might do this:
%:: touch $@
to cause all the source files needed (as prerequisites) to be created automatically.
You can instead define a recipe to be used for targets for which there are no rules at all, even ones which don't specify recipes. You do this by writing a rule for the target .DEFAULT. Such a rule's recipe is used for all prerequisites which do not appear as targets in any explicit rule, and for which no implicit rule applies. Naturally, there is no .DEFAULT rule unless you write one.
If you use .DEFAULT with no recipe or prerequisites:
For example, the following will take an unknown target and pass it down to the test directory's Makefile:
%:: $(MAKE) -C test $@