RFR: JDK-8035730 Configure fails in cygwin if current dir is in /home/user

[Magnus Ihse Bursie]

With the rewrite in JDK-8035495, we unfortunately lost the ability to 
run configure on Cygwin in the /home/<user> directory. (Since this is 
transcribed with cygpath to not begin with the usual /cygdrive/ prefix.)

This fix restores that functionality.
Bug: https://bugs.openjdk.java.net/browse/JDK-8035730
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8035730-configure-fails-in-cygwin-for-home-dir/webrev.01

/Magnus