Don't use extDirs in javadoc.

https://www.oracle.com/technetwork/java/javase/9-relnote-issues-3704069.html describes that in JDK 9, support for extdirs was removed. It causes an error with javac 11 that doesn't show up with standard Java on Mac OS X.

Author: Capstan

build.gradle

@@ -94,7 +94,6 @@ task sourcesJar(type: Jar, dependsOn: classes) {
 
 javadoc.options {
  overview = "src/main/javadoc/overview.html";
- extDirs = [ file("src/main/javadoc") ];
  docFilesSubDirs = true;
 }