RFR: JDK-8196356: Changes to m4 files don't trigger autoconf execution at build time

[Erik Joelsson]

The check for when to generate the generated configure script is not 
working quite as expected. It currently only compares timestamps if the 
local repository has any local changes in the make/autoconf directory. 
This used to make sense when we had a committed generated script, but 
now we actually do need to regenerate any time an input file is newer 
than the generated script.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196356
Webrev: http://cr.openjdk.java.net/~erikj/8196356/webrev.01/

/Erik