Ensure JDK 11 and 15 work on Mac OS

Test: tools/test.py --no-internal --test-namespace=ir/desugar/backports
Change-Id: Ic0ff3b810b30290a48ea07225e3e85bd5df2d043
diff --git a/build.gradle b/build.gradle
index 323256b..3e3b396 100644
--- a/build.gradle
+++ b/build.gradle
@@ -566,7 +566,9 @@
         if (OperatingSystem.current().isLinux()) {
             options.forkOptions.javaHome = file(jdkDir + 'linux')
         } else if (OperatingSystem.current().isMacOsX()) {
-            options.forkOptions.javaHome = file(jdkDir + 'osx')
+            options.forkOptions.javaHome = compatibility > JavaVersion.VERSION_1_9
+                    ? file(jdkDir + 'osx/Contents/Home')
+                    : file(jdkDir + 'osx')
         } else {
             options.forkOptions.javaHome = file(jdkDir + 'windows')
         }