RFR : 8015510 : (s) Improve JTReg location detection and provide location to test/Makefile

[Mike Duigou]

Hello all;

As a follow on to JDK-8007129 recently pushed to the jdk8/build repo I've improved the detection logic for finding jtreg. In addition to using the provided path it will also try the location specified by JT_HOME and in the command path. 

Attention people who just let magic happen and don't pay attention until something breaks: Currently if no location is defined for JT_HOME a default location of $(SLASH_JAVA)/re/jtreg/4.1/promoted/latest/binaries/jtreg is used. SLASH_JAVA is either /java on posix platforms or J:\\ on windows. This default location still works for now, but future changes will almost certainly remove it. You can avoid this pain by adding an appropriate --with-jtreg to your configure command today.

http://cr.openjdk.java.net/~mduigou/JDK-8015510/0/webrev/

If "--without-jtreg", "--with-jtreg=no" or no jtreg directive is used on the command line the spec.gmk vars JT_HOME and JTREGEXE will still present but will be empty. I am not sure if it's better (or possible) to avoid defining them in this case.

Also, since test/Makefile doesn't yet read spec.gmk it is necessary to provide it the JT_HOME location via an environment var in Main.gmk. This will eventually be fixed. 

Rather than wait for 8007129 to propagate to TL I would like someone to sponsor this change into build repo.

Thanks,

Mike