RFR: JDK-8073864 Configure must handle invalid elements on INCLUDE/LIB for visualstudio

[Magnus Ihse Bursie]

With the fix for JDK-8071329, the INCLUDE and LIB environment variables 
are parsed. Unfortunately, if they contain an empty element (e.g. starts 
with a ;) or an invalid path, the code breaks down.

This patch will skip empty elements and paths that do not exists.
Bug: https://bugs.openjdk.java.net/browse/JDK-8073864
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8073864-handle-invalid-include-lib-paths/webrev.01

/Magnus