[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.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3342 from ueshin/issues/SPARK-4429 and squashes the following commits:

14d86e8 [Takuya UESHIN] Add a comment.
4eef52b [Takuya UESHIN] Remove unneeded condition.
ce98d0f [Takuya UESHIN] Set non-empty value to system property "scala-2.11" if the property exists instead of adding profile.

project/SparkBuild.scala

@@ -101,14 +101,13 @@ object SparkBuild extends PomBuild {
       v.split("(\\s+|,)").filterNot(_.isEmpty).map(_.trim.replaceAll("-P", "")).toSeq
     }
 
-    if (profiles.exists(_.contains("scala-"))) {
-      profiles
-    } else if (System.getProperty("scala-2.11") != null) {
-      profiles ++ Seq("scala-2.11")
-    } else {
-      println("Enabled default scala profile")
-      profiles ++ Seq("scala-2.10")
+    if (System.getProperty("scala-2.11") == "") {
+      // To activate scala-2.11 profile, replace empty property value to non-empty value
+      // in the same way as Maven which handles -Dname as -Dname=true before executes build process.
+      // see: https://github.com/apache/maven/blob/maven-3.0.4/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java#L1082
+      System.setProperty("scala-2.11", "true")
     }
+    profiles
   }
 
   Properties.envOrNone("SBT_MAVEN_PROPERTIES") match {