Fix cleaning source libs

The clean script did delete the build.sh scripts. If the user reruns
the build scirpt, he can not fetch the sources and build again.

libraries/clean-source-libs.sh

@@ -14,7 +14,7 @@ echo "Cleaning bundled third-party dependencies..."
 if [ -e source/.svn ]; then
 	(cd source && svn revert -R . && svn st --no-ignore | cut -c 9- | xargs rm -rf)
 else
-	rm -rf source
+	find source/ ! -name '*build.sh*' --delete
 fi
 
 echo