Replace javah by javac -h, only write new constant files if something changes.

The const_generator changes help to ensure that e.g. Java rebuilds don't keep
rebuilding everything.

bindings/java/pom.xml

@@ -33,6 +33,12 @@
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.11.0</version>
+          <configuration>
+            <compilerArgs>
+              <arg>-h</arg>
+              <arg>target/headers</arg>
+             </compilerArgs>
+          </configuration>
         </plugin>
       </plugins>
     </pluginManagement>