RFR: JDK-8065215 Print warning summary at end of configure

[Magnus Ihse Bursie]

Warnings in configure tends to just fly by quickly and be buried in the 
verbose autoconf output.

We could with a simple trick repeat all those warnings (if any) at the 
end of configure, where they will be more easily spotted.

Bug: https://bugs.openjdk.java.net/browse/JDK-8065215
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8065215-print-warning-at-end-of-configure/webrev.01

/Magnus