Set buildNumber to 'unknown' when built out of git repository

pom.xml

@@ -301,11 +301,22 @@
             </goals>
             <configuration>
               <tasks>
-                <exec executable="git" outputproperty="buildNumber">
+                <taskdef resource="net/sf/antcontrib/antlib.xml" />
+                <exec executable="git" outputproperty="gitOutput" resultproperty="gitExitCode" failonerror="false" failifexecutionfails="false">
                   <arg value="log"/>
                   <arg value="-1"/>
                   <arg value="--format=format:%h"/>
                 </exec>
+                <if>
+                  <equals arg2="0" arg1="${gitExitCode}"/>
+                  <then>
+                    <property name="buildNumber" value="${gitOutput}"/>
+                  </then>
+                  <else>
+                    <property name="buildNumber" value="unknown"/>
+                  </else>
+                </if>
+                <echo>Build number: ${buildNumber}</echo>
                 <mkdir dir="${project.build.directory}" />
                 <echo message="${project.version}" file="${project.build.directory}/version.txt" />
                 <echo message="// DO NOT MODIFY - WILL BE OVERWRITTEN DURING THE BUILD PROCESS${line.separator}package org.jboss.netty.util;${line.separator}/**${line.separator} * Provides the version information of Netty.${line.separator} * @apiviz.landmark${line.separator} */${line.separator}public final class Version {${line.separator} /** The version identifier. */${line.separator} public static final String ID = &quot;${project.version}-${buildNumber}&quot;;${line.separator} /** Prints out the version identifier to stdout. */${line.separator} public static void main(String[] args) { System.out.println(ID); }${line.separator} private Version() { super(); }${line.separator}}${line.separator}" file="${basedir}/src/main/java/org/jboss/netty/util/Version.java" />