JDK 8 is now required as boot when building JDK 9

[Erik Joelsson]

Hello,

I just pushed [1] which makes configure require the boot jdk to be at 
least version 8 when building jdk 9. We usually require JDK N-1 to build 
JDK N and since we have had a number of issues related to older boot jdk 
versions, and since JDK 8 is generally available, it was time to make 
the change.

/Erik

[1] http://hg.openjdk.java.net/jdk9/dev/rev/79a52c1cc5b6