Drush generate-makefile needs to add "api = 2"

When a makefile is auto-generated with generate-makefile, drush make then complains it doesn't have the "api = 2" line. Drush make doesn't like the flavour of its own generate-makefile dogfood :)

I'm not sure what the long-term plan is for APIs, but I don't think that matters right now. A robust and easy fix, which makes things flexible for whatever the future might bring, is to make a define() statement and get everything in the codebase referencing that, including extra code in _drush_make_generate_makefile_contents(). In future we can look at getting the api number from a function call etc. but this gets us where generate-makefile needs to be.

Please find attached a patch that replaces all instances I've found of a check for api = 2 with DRUSH_MAKE_API, and adds that into the generate-makefile output.