Wrap jgit buildnumber plugin in a profile triggered by .git exsiting.

This should prevent numerous warnings when building from a source
archive instead of from git checkout.
diff --git a/pom.xml b/pom.xml
index 9c100e2..70348af 100644
--- a/pom.xml
+++ b/pom.xml
@@ -527,21 +527,6 @@
           <additionalparam>-header &apos;&lt;script type=&quot;text/javascript&quot; src=&quot;http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML&quot;&gt;&lt;/script&gt;&apos;</additionalparam>
         </configuration>
       </plugin>
-      <plugin>
-        <groupId>ru.concerteza.buildnumber</groupId>
-        <artifactId>maven-jgit-buildnumber-plugin</artifactId>
-        <version>${math.jgit.buildnumber.version}</version>
-        <executions>
-          <execution>
-            <phase>generate-resources</phase>
-            <goals>
-              <goal>extract-buildnumber</goal>
-            </goals>
-          </execution>
-        </executions>
-        <configuration>
-        </configuration>
-      </plugin>
     </plugins>
   </build>
 
@@ -693,6 +678,33 @@
 
   <profiles>
     <profile>
+      <id>jgit-buildnumber</id>
+      <activation>
+        <file>
+          <exists>.git</exists>
+        </file>
+      </activation>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>ru.concerteza.buildnumber</groupId>
+            <artifactId>maven-jgit-buildnumber-plugin</artifactId>
+            <version>${math.jgit.buildnumber.version}</version>
+            <executions>
+              <execution>
+                <phase>generate-resources</phase>
+                <goals>
+                  <goal>extract-buildnumber</goal>
+                </goals>
+              </execution>
+            </executions>
+            <configuration>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
+    <profile>
       <id>setup-checkout</id>
       <activation>
         <file>