Sometimes we need to re-run configure, due to changes in configure
files. Make detects such changes and forces a new run of configure.
If configure was run with a long command line, it's tricky to get this
right again. However, this can be done automatically by the Makefile.
By adding a new target 'reconfigure', if we have an existing
configuration, configure will be ran again with the same arguments as
last time.
Bug: https://bugs.openjdk.java.net/browse/JDK-8034199
WebRev:
http://cr.openjdk.java.net/~ihse/JDK-8034199-add-reconfigure-target/webrev.01
/Magnus
