Make some build properties extensions in buildSrc

They are used only in main build's .gradle.kts files, so it is more
flexible to define them as extensions - no need to republish
kotlin-build-gradle-plugin when another such property is to be added.

buildSrc/src/main/kotlin/BuildPropertiesExt.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+val KotlinBuildProperties.includeJava9: Boolean
+    get() = !isInJpsBuildIdeaSync && getBoolean("kotlin.build.java9", true)
+
+val KotlinBuildProperties.useBootstrapStdlib: Boolean
+    get() = isInJpsBuildIdeaSync || getBoolean("kotlin.build.useBootstrapStdlib", false)
+
+val KotlinBuildProperties.postProcessing: Boolean get() = isTeamcityBuild || getBoolean("kotlin.build.postprocessing", true)
+
+val KotlinBuildProperties.relocation: Boolean get() = postProcessing
+
+val KotlinBuildProperties.proguard: Boolean get() = postProcessing && getBoolean("kotlin.build.proguard", isTeamcityBuild)
+
+val KotlinBuildProperties.jarCompression: Boolean get() = getBoolean("kotlin.build.jar.compression", isTeamcityBuild)