Speeding up "empty" mx builds

[Doug Simon]

On one of my dev machines, I noticed that 'mx build’ took longer and longer to complete when no actual building is done. The culprit turned out to be the copying of graal.jar into each JDK variant in the workspace. I don’t yet know why the 
shutil.move python function takes so long, especially since the underlying drive is an SSD drive. As a workaround, I’ve added support[1] for using symlinks instead of copying graal.jar. On my machine the difference is:

[dsimon at coochie]~/linz/basic-graal$ time mx build

real	0m7.979s
user	0m1.832s
sys	0m0.736s
[dsimon at coochie]~/linz/basic-graal$ echo "SYMLINK_GRAAL_JAR=true" >> mx/env
[dsimon at coochie]~/linz/basic-graal$ time mx build

real	0m1.530s
user	0m1.704s
sys	0m0.376s

-Doug

[1] http://hg.openjdk.java.net/graal/graal/rev/97a10428ff2c