Add support for compile task specific settings on `compile` task axis

So far we had ability to configure compile task specific settings on
`Compile` Configuration scope only. We can now add `compile` task scope
as well.

Usecase: one can now specify:
```
// applies for all tasks on Compile axis
scalacOptions in Compile += "-deprecation"
// applies for compile tasks on Compile axis
scalacOptions in (Compile, compile) += "-Xcheckinit" // effectively, "-deprecation -Xcheckinit"
```

main/Defaults.scala

@@ -208,7 +208,7 @@ object Defaults extends BuildCommon
 	}
 	def compilersSetting = compilers <<= (scalaInstance, appConfiguration, streams, classpathOptions, javaHome) map { (si, app, s, co, jh) => Compiler.compilers(si, co, jh)(app, s.log) }
 
-	lazy val configTasks = docSetting(doc) ++ compileInputsSettings ++ Seq(
+	lazy val configTasks = docSetting(doc) ++ compileTaskSettings ++ compileInputsSettings ++ Seq(
 		initialCommands in GlobalScope :== "",
 		cleanupCommands in GlobalScope :== "",
 		compile <<= compileTask tag(Tags.Compile, Tags.CPU),
@@ -515,7 +515,9 @@ object Defaults extends BuildCommon
 				println()
 		}
 
-	def compileTask = (compileInputs, streams) map { (i,s) => Compiler(i,s.log) }
+	def compileTaskSettings: Seq[Setting[_]] = inTask(compile)(compileInputsSettings)
+
+	def compileTask = (compileInputs in compile, streams) map { (i,s) => Compiler(i,s.log) }
 	def compileIncSetupTask =
 		(dependencyClasspath, cacheDirectory, skip in compile, definesClass) map { (cp, cacheDir, skip, definesC) =>
 			Compiler.IncSetup(analysisMap(cp), definesC, skip, cacheDir / "inc_compile")

sbt/src/sbt-test/actions/compile/A.scala

@@ -0,0 +1,3 @@
+object A {
+	def x(i: Int) = 3
+}

sbt/src/sbt-test/actions/compile/B.scala

@@ -0,0 +1,3 @@
+object B {
+	def x(i: Int) = 3
+}

sbt/src/sbt-test/actions/compile/C.scala

@@ -0,0 +1,4 @@
+object C {
+	// broken method, D doesn't exist
+	def x: Int = D.i
+}

sbt/src/sbt-test/actions/compile/test

@@ -0,0 +1,5 @@
+-> compile
+
+> 'set sources in (Compile, compile) <<= sources in (Compile, compile) map { _.filterNot(_.getName contains "C") }'
+
+> compile