Validate required Java version in misc subproject.

Previously, validation of required version was done during loading of
build definition. This was needlessly restricting because not everything
in scalariform requires particular Java version. After closer inspection,
it turns out that only the `misc` subproject depends on Swing and cares
about the specific Java version.

We refactor validation logic to run right before `compile` task in `misc`
subproject. That makes it possible to ignore this subproject and build the
rest of scalariform on any Java version.

project/Build.scala

@@ -13,16 +13,15 @@ object ScalariformBuild extends Build {
 
    // This is to make sure nobody tries to compile with 1.6 as the target JDK.
    // Not clear if this will actually work on 1.8, needs to be tested when that is out.
-   val specVersion = sys.props("java.specification.version")
+   val validateJavaVersion = taskKey[Unit]("Check if we are running using required Java version")
    val mismatchedSpecificationMessage =
-   """|Java 1.7 is required for building Scalariform.
+   """|Java 1.7 is required for building the `misc` subproject of Scalariform.
       |
       |This is due to a dependency on the javax.swing library, which
       |had an API change from 1.6 to 1.7.
       |
       |Using 1.7 to build requires setting SBT to use JDK 1.7 or higher -- if SBT is
       |booting on JDK 1.6, you will get a javax.swing related compilation error.""".stripMargin
-   assert(specVersion == "1.7", mismatchedSpecificationMessage)
 
   lazy val commonSettings = Defaults.defaultSettings ++ SbtScalariform.defaultScalariformSettings ++ sonatypeSettings ++ Seq(
     organization := "com.danieltrinh",
@@ -109,6 +108,16 @@ object ScalariformBuild extends Build {
         "com.miglayout" % "miglayout" % "3.7.4"),
       publish := (),
       publishLocal := (),
+      validateJavaVersion := {
+        val specJavaVersion = sys.props("java.specification.version")
+        val compatibleJavaVersion = specJavaVersion == "1.7" || specJavaVersion == "1.8"
+        if (!compatibleJavaVersion)
+          sys.error(mismatchedSpecificationMessage)
+      },
+      // this means we'll validate required Java version only _right before_ running the compile
+      // command in misc subproject. In particular, build won't fail if user is not interested
+      // in building `misc` subproject.
+      compile in Compile := ((compile in Compile) dependsOn validateJavaVersion).value,
       mainClass in (Compile, run) := Some("scalariform.gui.Main"))) dependsOn (scalariform, cli)
 
   def pomExtraXml =