RFR: JDK-8069057 Make sure configure is run by bash

[Magnus Ihse Bursie]

It turns out that our effort to make sure the configure script is run by 
bash is not fool-proof.

The fix is to set CONFIG_SHELL ahead of calling the autoconf script.
Thanks to Dean Long for finding out the issue and testing the patch with 
dash.

Bug: https://bugs.openjdk.java.net/browse/JDK-8069057
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8069057-enforce-bash-in-configure/webrev.01

/Magnus