project.conf: split '--as-needed' in ldflags_defaults

Some projects have specific heuristics regarding '--as-needed' flag which can results in broken build. Splitting it should solve such issues.

• https://www.irccloud.com/pastebin/qHPgp67j/
• https://github.com/vim/vim/blob/4dbc2627641a6b950c30c31cbf7b7e6c36da1927/src/configure.ac#L4449