Comment 2 for bug 1387594

Ok, here are some more puzzling things :

The libnss-ldap from the archive does not use __libc_lock_lock :
$ pwd
./archive
$ wget http://fr.archive.ubuntu.com/ubuntu/pool/main/libn/libnss-ldap/libnss-ldap_264-2.2ubuntu4_amd64.deb
--2014-10-30 16:20:59-- http://fr.archive.ubuntu.com/ubuntu/pool/main/libn/libnss-ldap/libnss-ldap_264-2.2ubuntu4_amd64.deb
...
2014-10-30 16:21:01 (72,7 KB/s) - «libnss-ldap_264-2.2ubuntu4_amd64.deb» enregistré [64718/64718]
$ dpkg -x libnss-ldap_264-2.2ubuntu4_amd64.deb .
$ nm -D lib/x86_64-linux-gnu/libnss_ldap.so.2 | grep _lock
                 w __pthread_mutex_lock

The same source build in a PPA DOES have the symbol __libc_lock_lock :
$ cd ../ppa
$ pwd
./ppa
$ wget https://launchpad.net/~louis-bouchard/+archive/ubuntu/nssldap-buildtest/+files/libnss-ldap_264-2.2ubuntu4_amd64.deb
...
2014-10-30 16:23:56 (56,7 KB/s) - «libnss-ldap_264-2.2ubuntu4_amd64.deb» enregistré [83234/83234]

$ dpkg -x libnss-ldap_264-2.2ubuntu4_amd64.deb .
$ nm -D lib/x86_64-linux-gnu/libnss_ldap.so.2 | grep _lock
                 U __libc_lock_lock
                 U __libc_lock_unlock

I see the same behavior (i.e. fresh build created with __libc_lock_lock in use) when using sbuild or building manually using debian/rules

This needs more investigation