OPENJDK env var not playing well with new build

[Weijun Wang]

I thought building an OpenJDK or Oracle JDK can be fully controlled by 
using the --enable-openjdk-only configure option, but it seems the 
OPENJDK env variable still plays a part. If the variable is set to true 
and --enable-openjdk-only is not provided, the new build process gets 
confused and fails.

I understand this is my fault and I should not have that variable 
around. However, it will be nice if the build can deny all external 
variables like it did with all those ALT_*** variables. Are there any 
other variables I should be aware of?

Thanks
Max