[8u] RFR (S) 8065215: Print warning summary at end of configure

[Aleksey Shipilev]

Original RFE:
   https://bugs.openjdk.java.net/browse/JDK-8065215
   https://hg.openjdk.java.net/jdk9/jdk9/rev/eefa97d0d263

I would like to backport this 8u build system change to improve error detection during configure. 
This also allows cleaner backport of JDK-8079891.

The patch has a few minor differences:
   *) There is no CUSTOM_SUMMARY_AND_WARNINGS_HOOK, no configure.ac hunk does not apply automatically;
   *) There are no relevant "has been successfully created" blocks in help.m4;
   *) generated-configure.sh is obviously different;

8u webrev:
   https://cr.openjdk.java.net/~shade/8065215/webrev.8u.01/

Testing: 8u build; injecting synthetic AS_MSG_WARN into .m4 files shows the warning at the end of 
configure

-- 
Thanks,
-Aleksey