[SPARK-10952] Only add hive to classpath if HIVE_HOME is set.

Currently if it isn't set it scans `/lib/*` and adds every dir to the
classpath which makes the env too large and every command called
afterwords fails.

Author: Kevin Cox <kevincox@kevincox.ca>

Closes #8994 from kevincox/kevincox-only-add-hive-to-classpath-if-var-is-set.

build/sbt

@@ -20,10 +20,12 @@
 # When creating new tests for Spark SQL Hive, the HADOOP_CLASSPATH must contain the hive jars so
 # that we can run Hive to generate the golden answer.  This is not required for normal development
 # or testing.
-for i in "$HIVE_HOME"/lib/*
-do HADOOP_CLASSPATH="$HADOOP_CLASSPATH:$i"
-done
-export HADOOP_CLASSPATH
+if [ -n "$HIVE_HOME" ]; then
+    for i in "$HIVE_HOME"/lib/*
+    do HADOOP_CLASSPATH="$HADOOP_CLASSPATH:$i"
+    done
+    export HADOOP_CLASSPATH
+fi
 
 realpath () {
 (