Setup Kotlin compilation of java 9 source sets consistently

Usually, java 9 source sets contain only module-info.java file, so
Kotlin compilation does not run because there are no .kt sources.
However, it can still start after some incremental changes and
thus it's important to have its JVM target configured consistently
with Java compilation of the same source set.

buildSrc/src/main/kotlin/LibrariesCommon.kt

@@ -11,6 +11,7 @@ import org.gradle.api.file.FileCollection
 import org.gradle.api.tasks.compile.JavaCompile
 import org.gradle.kotlin.dsl.get
 import org.gradle.process.CommandLineArgumentProvider
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 @JvmOverloads
 fun Project.configureJava9Compilation(
@@ -19,6 +20,11 @@ fun Project.configureJava9Compilation(
 ) {
     configurations["java9CompileClasspath"].extendsFrom(configurations["compileClasspath"])
 
+    tasks.named("compileJava9Kotlin", KotlinCompile::class.java) {
+        configureTaskToolchain(JdkMajorVersion.JDK_9)
+        kotlinOptions.jvmTarget = JdkMajorVersion.JDK_9.targetName
+    }
+
     tasks.named("compileJava9Java", JavaCompile::class.java) {
         dependsOn(moduleOutputs)