gcc missing compile time check for _GLIBCXX_USE_NANOSLEEP

This enables a compile time check for availability of POSIX's
nanosleep(). Without this check, std::this_thread::sleep_for() and
std::this_thread::sleep_until() will not be available.

When compiling gcc, during ./configure it misses a check for _GLIBCXX_USE_NANOSLEEP. This is NOT enabled by default, which it should be (and it actually will be, see for example this discussion [1] ).


[1] http://stackoverflow.com/questions/12523122/what-is-glibcxx-use-nanosleep-all-about).