Use JDK 8 for compiling Java 5

Hopefully JDK 8 will actually be downloadable.

Author: DenWav

build.gradle.kts

@@ -176,15 +176,16 @@ tasks.withType<KotlinCompile>().configureEach {
 }
 
 // Compile classes to be loaded into the Gradle VM to Java 5 to match Groovy
-// This is for maximum compatibility, these classes will be loaded into every Gradle
-// import on all projects, so we don't want to break that with an incompatible class version.
+// This is for maximum compatibility, these classes will be loaded into every Gradle import on all
+// projects (not just Minecraft), so we don't want to break that with an incompatible class version.
 tasks.named(gradleToolingExtensionSourceSet.compileJavaTaskName, JavaCompile::class) {
-    val java7Compiler = javaToolchains.compilerFor { languageVersion.set(JavaLanguageVersion.of(7)) }
+    val java7Compiler = javaToolchains.compilerFor { languageVersion.set(JavaLanguageVersion.of(8)) }
     javaCompiler.set(java7Compiler)
     options.release.set(null as Int?)
     sourceCompatibility = "1.5"
     targetCompatibility = "1.5"
     options.bootstrapClasspath = files(java7Compiler.map { it.metadata.installationPath.file("jre/lib/rt.jar") })
+    options.compilerArgs = listOf("-Xlint:-options")
 }
 tasks.withType<GroovyCompile>().configureEach {
     options.compilerArgs = listOf("-proc:none")