Pull the Hadoop version from the environment.
Pull the Hadoop version from the environment.

This introduces two environment variables that you can use to control
the version of Hadoop that you build against. The Spark project
uses `SPARK_HADOOP_VERSION`, which is supported. By analogy, we
also support `SHARK_HADOOP_VERSION`, which has precedence over
`SPARK_HADOOP_VERSION`. If neither of these environment variables are
set at build time, the defaults from the build script are used.

The algorithm looks like this:

   1. Use `SHARK_HADOOP_VERSION` if set.
   2. Use `SPARK_HADOOP_VERSION` if set.
   3. Use the default that is hard-coded into the build definition.

mattyb149/blinkdb,mattyb149/blinkdb,mapr/shark,sameeragarwal/blinkdb,sjyk/sampleclean,alabid/blinkdb,sameeragarwal/blinkdb,amplab/shark,mapr/shark,mapr/shark,amplab/shark,sjyk/sampleclean,lzshlzsh/shark,alabid/blinkdb,alabid/blinkdb,lzshlzsh/shark,lzshlzsh/shark,amplab/shark,mattyb149/blinkdb,sjyk/sampleclean,sameeragarwal/blinkdb