RFR: JDK-8233034 configure needs to provide path to Xcode metal build tools

Mon Nov 4 13:29:17 UTC 2019

This is a patch to use configure to properly locate metal and metallib. 
This will make sure lanai is buildable by the Oracle internal CI system. 
This fix also cleans up how metal and metallib is called to use modern 
makefile patterns. This will e.g. improve logging of error messages.

It does not seem like you are using code review before push, but I 
thought I'd request one anyway. :) At least as an "It's ok, go ahead and 
push".

Bug: https://bugs.openjdk.java.net/browse/JDK-8233034
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8233034-lanai-metal-build-fix/webrev.01

/Magnus