build: Add Xcode version check (#3543)

gradle.properties

@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 
-# A version of the Kotlin "toolchain" (compiler, runtime, stdlib etc) used by the build script.
+# A version of the Kotlin compiler that is used to build Kotlin/Native.
 buildKotlinVersion=1.3.70-dev-1070
 buildKotlinCompilerRepo=https://teamcity.jetbrains.com/guestAuth/app/rest/builds/buildType:(id:Kotlin_KotlinPublic_Compiler),number:1.3.70-dev-1070,branch:default:any,pinned:true/artifacts/content/maven
 remoteRoot=konan_tests
@@ -25,6 +25,10 @@ kotlinStdlibVersion=1.3.70-dev-1244
 kotlinStdlibTestsVersion=1.3.70-dev-1244
 testKotlinCompilerVersion=1.3.70-dev-1244
 konanVersion=1.3.70
+
+# A version of Xcode required to build the Kotlin/Native compiler.
+xcodeMajorVersion=11
+
 org.gradle.jvmargs='-Dfile.encoding=UTF-8'
 org.gradle.workers.max=4