Making it easy to activate the animal-sniffer Java5 check.

I originally attempted to define this in a profile that can be activated
from a child POM, but looking at MNG-3228
<http://jira.codehaus.org/browse/MNG-3228>, it appears that such use of
profiles aren't possible.

So I'm falling back to the <pluginManagement>, which can be activated
via the following fragment in build/plugins/plugin:

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>animal-sniffer-maven-plugin</artifactId>
  <!-- details defined in the parent POM -->
</plugin>

plugins/pom.xml

@@ -86,16 +86,15 @@
       <version>2.4</version>
       <scope>provided</scope>
     </dependency>
+    <dependency>
+      <!-- for JRE requirement check annotation -->
+      <groupId>org.codehaus.mojo</groupId>
+      <artifactId>animal-sniffer-annotations</artifactId>
+      <version>1.7</version>
+      <scope>provided</scope>
+      <optional>true</optional><!-- no need to have this at runtime -->
+    </dependency>
   </dependencies>
-  <!--dependencyManagement>
-       <dependencies>
-       <dependency>
-       <groupId>org.jenkins-ci.main</groupId>
-       <artifactId>maven-plugin</artifactId>
-       <version>1.395</version>
-       </dependency>
-       </dependencies>
-       </dependencyManagement-->
 
   <build>
     <!--
@@ -250,6 +249,27 @@
             </additionalProjectnatures>
           </configuration>
         </plugin>
+        <plugin>
+          <!-- make sure our code doesn't have 1.6 dependencies except where we know it -->
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>animal-sniffer-maven-plugin</artifactId>
+          <version>1.7</version>
+          <executions>
+            <execution>
+              <goals>
+                <goal>check</goal>
+              </goals>
+              <phase>test</phase>
+              <configuration>
+                <signature>
+                  <groupId>org.codehaus.mojo.signature</groupId>
+                  <artifactId>java15</artifactId>
+                  <version>1.0</version>
+                </signature>
+              </configuration>
+            </execution>
+          </executions>
+        </plugin>
       </plugins>
     </pluginManagement>
   </build>