After make clean, at least these files stay behind:
./src/libbitcoin_consensus.a
./src/libbitcoin_wallet.a
./src/libbitcoin_common.a
./src/libbitcoin_cli.a
./src/crypto/libbitcoin_crypto.a
./src/libbitcoin_util.a
./src/libbitcoin_server.a
./src/qt/libbitcoinqt.a
(there may be more, but I happened to notice these ones - leaving libraries behind would easily result in conflicts)