Doing a quick google search, you'll find JavaScriptCore is under the LGPL, which is a sort of non-virile version of the GPL (although you should probably consult with a lawyer about this - I am not one).
If that doesn't work, there are a bunch of other implementations of JavaScript out there - Mozilla Rhino is another good one.
If you are trying to do a language from scratch, one thing I would recommend looking into is Scala's "Parser Combinators" (just do a google search for "scala parser combinator"). It lets you create a parser pretty easily in the same language that your coding the processing of the syntax tree in, which is nice.