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

[Tim Bell]

Magnus:

> 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

Looks good.

Tim