Define default build JVM toolchain

buildSrc/src/main/kotlin/JvmToolchain.kt

@@ -33,8 +33,10 @@ enum class JdkMajorVersion(
     }
 }
 
+val DEFAULT_JVM_TOOLCHAIN = JdkMajorVersion.JDK_1_8
+
 fun Project.configureJvmDefaultToolchain() {
-    configureJvmToolchain(JdkMajorVersion.JDK_1_8)
+    configureJvmToolchain(DEFAULT_JVM_TOOLCHAIN)
 }
 
 fun Project.shouldOverrideObsoleteJdk(

buildSrc/src/main/kotlin/dependencies.kt

@@ -295,7 +295,7 @@ fun Project.toolsJarApi(): Any =
         dependencies.project(":dependencies:tools-jar-api")
 
 fun Project.toolsJar(): FileCollection = files(
-    getToolchainLauncherFor(JdkMajorVersion.JDK_1_8)
+    getToolchainLauncherFor(DEFAULT_JVM_TOOLCHAIN)
         .map {
             Jvm.forHome(it.metadata.installationPath.asFile).toolsJar ?: throw GradleException("tools.jar not found!")
         }