[patch] Default to --enable-libstdcxx-time=auto

[Paolo Carlini]

.. put an abort or something in the dummy implementations, to make sure 
people don't try to run an executable built with headers which have the 
clock available linked at run time to a .so which doesn't, really. 
Suboptimal solution because ideally we would like to fail at build time, 
but this is the best I have got so far. Be very clear in the docs, anyway.

Paolo.