Code review request: 7142426: [macosx] cannot build if the not-yet-build jdk is in PATH

[Weijun Wang]

The rungen script has not forced the use of java in $JAVA_HOME. Since I 
always put the to-be-built java at the top of $PATH, the script fails 
because that java exists but its runtime is still not fully built.

Please take a look at my fix

    http://cr.openjdk.java.net/~weijun/7142426/webrev.00/

Also, I'm not sure where runjava is used. If it's really useless, we can 
remove the file, as well as the reference in jdk/make/java/jobjc/Makefile.

The bug was closed by RE but I'll re-open it if you believe this is a 
real bug.

Thanks
Max