Need reviewer for fix to Mac jdk partial builds

[Kelly O'Hair]

7191703: jprt cannot build jdk on MacOSX.
http://cr.openjdk.java.net/~ohair/openjdk8/jobjc-fix/webrev/

Fixes a partial build issue with the jdk repo on the Mac.
If LANGTOOLS_DIST is defined, nothing changes and the ant script should find langtools/dist/classes.jar.
Otherwise, by running ant with the import jdk, it should build ok.

I very much dislike this logic but don't feel it is worth doing more than the minimum here.

-kto