RFR: JDK-8138739: Enable devkit on macosx in JPRT (again)

[Erik Joelsson]

Hello,

Please review this minor fix to the jprt configuration, which again 
enables the use of devkits for macosx.

This will be pushed to jdk9/client as that's where the final fixes 
needed for this are gathered.

Bug: https://bugs.openjdk.java.net/browse/JDK-8138739
Patch:
diff -r 1e64ec3d1c60 make/jprt.properties
--- a/make/jprt.properties
+++ b/make/jprt.properties
@@ -127,6 +127,8 @@
      ${jprt.i586.build.configure.args}
  jprt.windows_x64.build.configure.args=                    \
      --with-devkit=$VS2013_HOME
+jprt.macosx_x64.build.configure.args=                    \
+    --with-devkit=$XCODE_511_HOME

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