Can't build Confluence 5.1 from source code

after installing a few external not provided jars, the build of ./build.sh fails with

[...]
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Failed to resolve artifact.

Missing:
----------
1) com.atlassian.confluence:confluence-testutils:jar:5.1-SNAPSHOT

Try downloading the file manually from the project website.

Then, install it using the command:
mvn install:install-file -DgroupId=com.atlassian.confluence -DartifactId=confluence-testutils -Dversion=5.1-SNAPSHOT -Dpackaging=jar -Dfile=/path/to/file

Alternatively, if you host your own repository you can deploy the file there:
mvn deploy:deploy-file -DgroupId=com.atlassian.confluence -DartifactId=confluence-testutils -Dversion=5.1-SNAPSHOT -Dpackaging=jar -Dfile=/path/to/file -Durl=[url] -DrepositoryId=[id]

Path to dependency:
1) com.atlassian.confluence:confluence-sanity-tests:jar:5.1
2) com.atlassian.confluence:confluence-testutils:jar:5.1-SNAPSHOT

----------
1 required artifact is missing.

for artifact:
com.atlassian.confluence:confluence-sanity-tests:jar:5.1

from the specified remote repositories:
central (http://repo1.maven.org/maven2),
atlassian-contrib (https://m2proxy.atlassian.com/contrib),
atlassian-proxy (https://m2proxy.atlassian.com/repository/public)