Make JDK 9 required to build the project

ReadMe.md

@@ -29,15 +29,16 @@ Welcome to [Kotlin](https://kotlinlang.org/)! Some handy links:
 
 In order to build Kotlin distribution you need to have:
 
-- JDK 1.6, 1.7 and 1.8
+- JDK 1.6, 1.7, 1.8 and 9
 - Setup environment variables as following:
 
         JAVA_HOME="path to JDK 1.8"
         JDK_16="path to JDK 1.6"
         JDK_17="path to JDK 1.7"
         JDK_18="path to JDK 1.8"
+        JDK_9="path to JDK 9"
 
-For local development, if you're not working on bytecode generation or the standard library, it's OK to have only JDK 8 installed, and to point all of the environment variables mentioned above to your JDK 8 installation.
+For local development, if you're not working on bytecode generation or the standard library, it's OK to have only JDK 1.8 and JDK 9 installed, and to point JDK_16 and JDK_17 environment variables to your JDK 1.8 installation.
 
 You also can use [Gradle properties](https://docs.gradle.org/current/userguide/build_environment.html#sec:gradle_properties_and_system_properties) to setup JDK_* variables.
 

build.gradle.kts

@@ -117,7 +117,7 @@ Properties().apply {
 extra["JDK_16"] = jdkPath("1.6")
 extra["JDK_17"] = jdkPath("1.7")
 extra["JDK_18"] = jdkPath("1.8")
-extra["JDK_9"] = jdkPathIfFound("9")
+extra["JDK_9"] = jdkPath("9")
 extra["JDK_10"] = jdkPathIfFound("10")
 
 rootProject.apply {