When there's a dependency on a task that does nothing, -j2 reexecutes target anyway

[FooBarWidget]

The problem is best explained with a test case:

file 'foo.txt' => :libev do
    sh 'touch foo.txt'
end

task :libev => 'libev.txt'

file 'libev.txt' do
    sh 'touch libev.txt'
end

Run:

$ rake foo.txt
touch libev.txt
touch foo.txt
$ rake foo.txt     # Does nothing, as expected.
$ drake foo.txt    # Does nothing, as expected.

But:

$ drake -j2 foo.txt
touch foo.txt

Phusion Passenger depends on this pattern: we have a :libev task which depends on libev.a, or does nothing, depends on whether the user has set the USE_VENDORED_LIBEV flag.