check if we need to link pthread dynamically

On Linux, bionic and musl for instance do not need libpthread. With
bionic, there is not even a stub library for compatibility.

m4/check-libc.m4

@@ -130,6 +130,7 @@ AC_CHECK_FUNCS([getauxval])
 AC_SEARCH_LIBS([dl_iterate_phdr],[dl])
 AC_CHECK_FUNCS([dl_iterate_phdr])
 
+AC_SEARCH_LIBS([pthread_once],[pthread])
 AC_SEARCH_LIBS([clock_gettime],[rt posix4])
 AC_CHECK_FUNCS([clock_gettime])
 AM_CONDITIONAL([HAVE_CLOCK_GETTIME], [test "x$ac_cv_func_clock_gettime" = xyes])