Make all pthread symbols weak and test before calling any of them.  This should allow us to work on platforms like GNU/Linux that don't export pthread stubs in libc.
Make all pthread symbols weak and test before calling any of them.  This should allow us to work on platforms like GNU/Linux that don't export pthread stubs in libc.

vegard/libcxxrt,cemeyer/libcxxrt,xin3liang/platform_external_libcxxrt,sudosurootdev/external_libcxxrt,vegard/libcxxrt,sudosurootdev/external_libcxxrt,xin3liang/platform_external_libcxxrt,cemeyer/libcxxrt,vegard/libcxxrt,xin3liang/platform_external_libcxxrt,sudosurootdev/external_libcxxrt,cemeyer/libcxxrt