Add libpthread to library list

Add libpthread to the library list, so it will be directly linked in
both the net plugin and executables.  The plugin uses pthreads, so
should have been including this by default.  We got lucky because
the pthread calls were in something we were building as a .so and
not in the unit tests.  But breaking that in a future commit breaks
the getting lucky part of the equation.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

configure.ac

@@ -61,6 +61,7 @@ AC_FUNC_MALLOC
 AC_CHECK_FUNC([memset], [], [AC_MSG_ERROR([NCCL OFI Plugin requires memset function.])])
 AC_CHECK_FUNC([realpath], [], [AC_MSG_ERROR([NCCL OFI Plugin requires realpath function.])])
 AC_SEARCH_LIBS([dlopen], [dl], [], [AC_MSG_ERROR([NCCL OFI Plugin requires dlopen])])
+AC_SEARCH_LIBS([pthread_getspecific], [pthread], [], [AC_MSG_ERROR([NCCL OFI Plugin requires pthreads.])])
 
 # Check for GCC builtin functions
 CHECK_GCC_BUILTIN([__builtin_expect])