[Xalan] build with JDK 9

Though can run with Java 9, building Xalan still needs JDK 8 (or 7). Building with JDK 9 gives the following error:

serializer.compile:
     [echo] Compiling the Serializer
    [javac] /Users/johnz/Repos/dacapobench/benchmarks/bms/xalan/build/xalan-j_2_7_2/build.xml:1690: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds
    [javac] Compiling 83 source files to /Users/johnz/Repos/dacapobench/benchmarks/bms/xalan/build/xalan-j_2_7_2/build/serializer
    [javac] Fatal Error: Unable to find package java.lang in classpath or bootclasspath

BUILD FAILED