RFR: JDK-8252769 Fail-fast in configure if git config autocrlf has invalid value

[Magnus Ihse Bursie]

Over and over again, people are running into issues when their git 
config has core.autocrlf set to true or input. (This is apparently done 
by default by some Windows git installations.)

This will not work with the OpenJDK build system.
We should check this at configure time and give suitable error message 
if this is detected.

Bug: https://bugs.openjdk.java.net/browse/JDK-8252769
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8252769-warn-on-autocrlf/webrev.01

/Magnus