Cross-building with and without cross-building spec in project/Build.scala

gistfile1.txt

13:57:47 cees@Cees-MBP:~/OpenSource/unclever$ sbt "+ package"
Listening for transport dt_socket at address: 5005
[info] Loading project definition from /Users/cees/OpenSource/unclever/project
[info] Set current project to unclever (in build file:/Users/cees/OpenSource/unclever/)
[info] Setting version to 2.10.5
[info] Reapplying settings...
[info] Set current project to unclever (in build file:/Users/cees/OpenSource/unclever/)
[info] Packaging /Users/cees/OpenSource/unclever/target/scala-2.10/unclever_2.10-0.1-SNAPSHOT.jar ...
[info] Done packaging.
[success] Total time: 0 s, completed Aug 24, 2015 1:57:58 PM
[info] Setting version to 2.11.7
[info] Reapplying settings...
[info] Set current project to unclever (in build file:/Users/cees/OpenSource/unclever/)
[info] Packaging /Users/cees/OpenSource/unclever/target/scala-2.11/unclever_2.11-0.1-SNAPSHOT.jar ...
[info] Done packaging.
[success] Total time: 0 s, completed Aug 24, 2015 1:57:59 PM
[info] Setting version to 2.11.7
[info] Reapplying settings...
[info] Set current project to unclever (in build file:/Users/cees/OpenSource/unclever/)
13:57:59 cees@Cees-MBP:~/OpenSource/unclever$ cd ../hardcoded/
13:58:07 cees@Cees-MBP:~/OpenSource/hardcoded$ sbt "+ package"
Listening for transport dt_socket at address: 5005
[info] Loading project definition from /Users/cees/OpenSource/hardcoded/project
[info] Set current project to hardcoded (in build file:/Users/cees/OpenSource/hardcoded/)
[info] Setting version to 2.10.4
[info] Reapplying settings...
[info] Set current project to hardcoded (in build file:/Users/cees/OpenSource/hardcoded/)
[info] Updating {file:/Users/cees/OpenSource/hardcoded/}hardcoded...
[info] Resolving org.fusesource.jansi#jansi;1.4 ...
[info] Done updating.
[warn] Scala version was updated by one of library dependencies:
[warn] 	* org.scala-lang:scala-library:2.10.4 -> 2.10.5
[warn] To force scalaVersion, add the following:
[warn] 	ivyScala := ivyScala.value map { _.copy(overrideScalaVersion = true) }
[warn] Run 'evicted' to see detailed eviction warnings
[info] Compiling 1 Scala source to /Users/cees/OpenSource/hardcoded/target/scala-2.10/classes...
[info] Packaging /Users/cees/OpenSource/hardcoded/target/scala-2.10/hardcoded_2.10-0.1-SNAPSHOT.jar ...
[info] Done packaging.
[success] Total time: 4 s, completed Aug 24, 2015 1:58:21 PM
[info] Setting version to 2.10.5
[info] Reapplying settings...
[info] Set current project to hardcoded (in build file:/Users/cees/OpenSource/hardcoded/)

Unclever specifies two cross-build versions (https://github.com/cdegroot/unclever/blob/master/project/Build.scala#L10). Hardcoded does not have that line (https://github.com/cdegroot/hardcoded/blob/master/project/Build.scala). In both cases, sbt "+ packages" does what you want - build the indicated versions.