[SPARK-4429][BUILD] Build for Scala 2.11 using sbt fails.

I tried to build for Scala 2.11 using sbt with the following command:

```
$ sbt/sbt -Dscala-2.11 assembly
```

but it ends with the following error messages:

```
[error] (streaming-kafka/*:update) sbt.ResolveException: unresolved dependency: org.apache.kafka#kafka_2.11;0.8.0: not found
[error] (catalyst/*:update) sbt.ResolveException: unresolved dependency: org.scalamacros#quasiquotes_2.11;2.0.1: not found
```

The reason is:
If system property `-Dscala-2.11` (without value) was set, `SparkBuild.scala` adds `scala-2.11` profile, but also `sbt-pom-reader` activates `scala-2.10` profile instead of `scala-2.11` profile because the activator `PropertyProfileActivator` used by `sbt-pom-reader` internally checks if the property value is empty or not.

The value is set to non-empty value, then no need to add profiles in `SparkBuild.scala` because `sbt-pom-reader` can handle as expected.
Could you add a comment here and link to the maven code this is replicating? It might be tricky for other developers to understand why this exists.