additional parameters for zc.recipe.egg:custom
Some setup scripts require additional parameters for building an egg. (most probably only eggs with external libraries).
The attached patch adds an additional 'build-ext' option to the Custom recipe of zc.recipe.egg.
The patch recognises each line in a given 'build-ext' option as additional parameter for build_ext. If an option contains a '=' then the option is split and passed down to build_ext accordingly.