By default, all paths in the build config (e.g. externs) are assumed to be relative to the ol3 repo root. To make it so relative paths are resolved from a different directory, use the cwd option.
In addition, this change makes it so the `build.js` task can be run from another directory.
This allows users to build ol3 without anything exposed in the
global namespace. This can e.g. be useful for creating an ol3
AMD module, by simply using a build configuration with
"define('ol',function(){var o={};%output%return o.ol;});" as
"output_wrapper".
Whitespace builds are still painful to debug. Skipping the compiler altogether and simply concatenating sources is a better option for development and debugging. By ommitting the `compile` option in a build config, the output is "uncompiled" - a straight concatenation of all sources in dependency order.
