RFR: JDK-8079891 Store configure log in $BUILD/configure.log

[Magnus Ihse Bursie]

It is trivial to save the log file of configure in the build output 
directory. We should do that.

I also noted that the repeat of warnings at the end of the configure run 
had stopped working since we moved config.log to configure-support. 
(Also, it was incorrectly indented.) I fixed this as well.

After a successful configure run, build/$BUILD/configure.log will be 
created, and contain exactly what was printed to stdout/stderr during 
the configure execution. In case of a repeated run of configure, the old 
log will be rotated to configure.log.old before being deleted, similar 
to how build.log works. If configure fails before completion, the 
configure.log file will be left in the current directory -- same as the 
current autoconf artifacts.

This code utilized the functionality already present for the flag 
--debug-configure. This means that this flag slightly changes meaning. 
It will log configure output as before, but to the new name 
(configure.log). The only effect of this flag now is to change the 
"debug level" of configure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8079891
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8079891-save-configure-log/webrev.01

/Magnus