"docs-reference" target fails with `extlink.spec.version` system property not set. It's a regression caused by JDK-8205627. docs-reference is built with BootJDK javadoc that also uses ExtLink taglet and so make/Docs.gmk needs to be adjusted to set the system property in $JAVADOC_CMD.
The fix is straight-forward (thanks to Erik for suggesting this better fix). http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8206184/webrev.00/ Mandy
