Running 'make clean-all' has errors if a dep has no Makefile

Create a project with a dependency that has no Makefile such as wooga/eredis.
Do 'make all' then 'make clean-all' to get an error. This patch checks for the
presence of the Makefile before attempting to run 'make clean', however, it does
not attempt run 'rebar clean' or other commands instead.
Merge branch 'master' of git://github.com/jaynel/erlang.mk