RFR: JDK-8196365: Overriding configure variable from custom hook generates warning

[Tim Bell]

Erik:

> When running configure, you can override any tool location by setting
> the variable name on the command line (e.g. configure
> MAKE=/path/to/make). Configure also includes validation that any such
> argument actually corresponds to a valid variable and will print a
> warning if no match is found.
>
> This validation logic is currently evaluated before the CUSTOM_LATE_HOOK
> is called so any variable defined there will still trigger the warning
> if specified on the command line. This patch moves the validation to
> after CUSTOM_LATE_HOOK.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8196365
>
> Webrev: http://cr.openjdk.java.net/~erikj/8196365/webrev.01/index.html

Looks good.

/Tim