Fix a problem where all classes are compiled again

Motivation:

Due to a known problem[1] of maven-compiler-plugin, our build always
compiles everything from scratch, which is waste of time.

Modifications:

Exclude package-info.java from the source list.

Result:

Much shorter build time.

[1]: https://jira.codehaus.org/browse/MCOMPILER-205

pom.xml

@@ -624,6 +624,9 @@
           -->
           <meminitial>256m</meminitial>
           <maxmem>1024m</maxmem>
+          <excludes>
+            <exclude>**/package-info.java</exclude>
+          </excludes>
         </configuration>
       </plugin>
       <plugin>