RFR: 8007129: build-infra Add configure --with-jtreg option for location of JTREG

[Tim Bell]

Erik:

> This patch adds a configure option --with-jtreg=/path/to/jtreg.
>
> It also changes how BASIC_FIXUP_PATH works on unix platforms, making 
> it behave more like it's documented and like on windows. Relative 
> paths are made absolute and special variables like ~ are expanded. 
> This macro didn't have a lot of uses that weren't windows specific so 
> I doubt it will affect anything in a bad way.
>
> http://cr.openjdk.java.net/~erikj/8007129/webrev.root.01

Looks good to me.

Tim