libcdecl: Punt gnulib lock module.

libcdecl: Punt gnulib lock module.

This module is total overkill for what we need, which is to provide
one-time library initialization.  Except for the pthread_in_use
detection, this module does not appear to work around any particular
portability problem with POSIX pthread_once or standard C call_once.

On the other hand, on GNU/Linux the unused code this module links in
represents almost 5% of the size of the entire library (which includes
eight pointless PLT entries).

We have to make a local copy of the pthread_in_use macro definition,
as it is no longer imported into the project.  Further improvements
may be possible to specialize this test for our use case, but for
now stick with the Gnulib version which is known to work on a lot
of different systems.