Use File.pathSeparator instead of File.separator when building boot classpath

When building on my mac I get this error:

    Fatal Error: Unable to find package java.lang in classpath or bootclasspath

The source seems to be that the bootclasspath is being concatenated using `File.separator`, which is a slash on mac. This leads to a classpath like this: `/usr/blah/blah.jar//usr/blah/qwer.jar//usr/blah/zxcv.jar`.

Using `File.pathseparator`, which is a colon on mac, you get a classpath like this: `/usr/blah/blah.jar:/usr/blah/qwer.jar:/usr/blah/zxcv.jar`, which java seems to like better.