RFR: JDK-8189955 Configuration validation is broken for some types of paths

[Magnus Ihse Bursie]

We are validating that the topdir we're using when running make is the 
same as when we were running configure.

For some circumstances, most typically when using a subdirectory in the 
user's home directory (e.g. ~/jdk10) on Cygwin, this check fails.

Bug: https://bugs.openjdk.java.net/browse/JDK-8189955
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8189955-configuration-validation-is-broken-for-some-paths/webrev.01

/Magnus