[jdk10 & jdk8u] Request for Review: 8173654

[Rob McKenna]

Hi folks,

We've found it useful to be able to override LIBJAVA_SRC_DIRS /
LIBJAVA_MAPFILE in libjava. This change in 10 allows the former and
backports the latter to 8:

http://cr.openjdk.java.net/~robm/8173654/

Thanks,

    -Rob