Correctly build projects that depend through exported dependencies.

The Scala builder optimised the case where the builder was triggered with
no changes in a project, by skipping the call to the underlying Sbt builder.
This is wrong when the project has no changes itself, but a dependent project has.

Fixed #1001904.

Consider for back porting to 3.0.x(cherry picked from commit 8eeb650f70f68d3804cf93e6ad07ae30bbff7e3c)

Conflicts:

	org.scala-ide.sdt.core/src/scala/tools/eclipse/buildmanager/sbtintegration/EclipseSbtBuildManager.scala