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

[Erik Joelsson]

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

/Erik