Set source and target compatibility in buildSrc

If source and target compatibility are not set explicity,
Gradle will assume they are equal to current JVM version.
This may cause unwanted rebuilds, when a build is run on
different JDK versions (it might happen when switching between
CLI and IDE).

buildSrc/build.gradle.kts

@@ -112,6 +112,11 @@ samWithReceiver {
 fun Project.`samWithReceiver`(configure: org.jetbrains.kotlin.samWithReceiver.gradle.SamWithReceiverExtension.() -> Unit): Unit =
         extensions.configure("samWithReceiver", configure)
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 tasks["build"].dependsOn(":prepare-deps:build")
 
 allprojects {