RFR: JDK-8199640 Split up BUILD_LIBKRB5 into the two, unrelated compilations it consists of

Wed Mar 14 23:18:10 UTC 2018

BUILD_LIBKRB5 is currently a strange chimera between compiling 
w2k_lsa_auth on windows, and osxkrb5 on macos. They do not share source 
code, name or compilation options.

This patch separates them into two separate compilations, one for each 
platform, so that they then can follow the normal JDK pattern for 
compiling libs.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199640
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8199640-split-up-BUILD_LIBJRB5/webrev.01

/Magnus