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

[Erik Joelsson]

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/

/Erik