Fix java.ext.dirs path on OS X

As reported by @cmci [on the ImageJ mailing list](https://list.nih.gov/cgi-bin/wa.exe?A2=IMAGEJ;5de8bddc.1407), the `java.ext.dirs` system property is set to `ImageJ.app/java/macosx-java3d/Home/lib/ext/Library/Java/Extensions` rather than `Fiji.app/java/macosx-java3d/Home/lib/ext` as intended, which causes problems with the local Java3D installation.