implementation of std::thread::hardware_concurrency()

[Jonathan Wakely]

This provides a working thread::hardware_concurrency on platforms that
support pthread_num_processors_np or the "hw.ncpu" sysctl, but by
testing for the features in configure rather than hardcoding OS macro
tests in thread.cc

        * acinclude.m4 (GLIBCXX_CHECK_SC_NPROC_ONLN): Define.
        (GLIBCXX_CHECK_PTHREADS_NUM_PROCESSORS_NP): Define.
        (GLIBCXX_CHECK_SYSCTL_HW_NCPU): Define.
        * configure.ac: Use new checks.
        * configure: Regenerate.
        * config.h.in: Regenerate.
        * src/thread.cc: Check new config macros.
        * testsuite/lib/libstdc++.exp: Likewise.

Tested x86_64-linux and x86_64-netbsd5.1, committed to trunk
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