libmemcached: fix build "tr1/cintypes" error on OS X 10.9

A fix for the "tr1/cintypes" build issue on OS X 10.9.
Fixes #20625.

**Regarding #20635**:
It actually seems to be expected, that the tr1 headers don't exist when using clang.

I was told:
* The tr1 headers were an experiment by the C++ standards committee.  They are not part of any standard.
* On Mavericks libc++ has become the default used by the compiler clang.
* The newer libc++ lacks the tr1 experiment.  However most of tr1 was standardized with C++11, and now lives in namespace std, and in headers not prefixed with tr1/.