Allow out of tree builds.

Before modularisation of the code, it was possible to do an out of tree build
giving an -PbuildDir=someDir argument to gradle. With the modularisation, it's
broken as using -PbuildDir=someDir will force the same directory for each
projects, which breaks the gradle build system.

This commit adds a new externalBuildDir project property to allow out of tree
builds again. When set, it updates the buildDir property for each project to
point to ${externalBuildDir}/${project.name}. That way, the build artifacts are
written in the out of tree directory in a project specific folder.

To do an out of tree build, use

    gradle -PexternalBuildDir=someDir ...

It supports absolute and relative path. Relative path are interpreted against
the ExoPlayer root directory.

build.gradle

@@ -51,6 +51,11 @@ allprojects {
         releaseVersion = 'r2.3.1'
         releaseWebsite = 'https://github.com/google/ExoPlayer'
     }
+    if (it.hasProperty('externalBuildDir')) {
+        if (!new File(externalBuildDir).isAbsolute())
+            externalBuildDir = new File(rootDir, externalBuildDir)
+        buildDir = "${externalBuildDir}/${project.name}"
+    }
 }
 
 def getBintrayRepo() {