Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56268
Merged revision 1578817 from tomcat/trunk:
Specify the default value of the ${build} in the build section.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1578818 13f79535-47bb-0310-9956-ffa450edef68