RFR: JDK-8031759 Configure should handle overrides of tools better

[Tim Bell]

Magnus:

> Okay, so here's the revised version. What's new this time:
>
> * Overriding of tools is only supported on the command line. If user 
> types e.g. "CAT=miaow configure", this will result in a warning (and 
> cat being detected normally). "configure CAT=/bin/woof", otoh, will 
> use the provided value instead of searching. (And "configure 
> CAT=missing" will search the $PATH for a binary named "missing", and 
> use that if found).
>
> * If the user specifies an override that does not match a real tool, 
> e.g. "configure NOSUCHTOOL=broken", then the configure script will 
> report this as an incorrect argument error.
>
> * Variables that can be overrided in this manner are published in the 
> help, using AC_ARG_VAR (thanks Mike for the idea!)
>
> * I have added the greadlink alternative for READLINK from Mike's patch.
>
> * The tool setup code is considerably rewritten, to collect all actual 
> work in a "workhorse" function, with thin wrappers providing the 
> external API. This enabled removing of some unneccessary code 
> duplication.
>
> WebRev: 
> http://cr.openjdk.java.net/~ihse/JDK-8031759-better-overriding-of-tools-in-configure/webrev.03

Looks good to me.

Tim