Not thread safe! But I don't think we care.
We want to evaluate the env DSL block in the context of ENV for asthetic
reasons, but we also want access to methods on the requirement instance.
We can use #instance_exec to pass the requirement itself into the block:
class Foo < Requirement
env do |req|
append 'PATH', req.some_path
end
def some_path
which 'something'
end
end
Also add a simplified version of Object#instance_exec for Ruby 1.8.6.
