We need to test linkage of pthread_create and pthread_join,
as pthread_mutex_* and pthread_key_* functions do not need
extra linkage under FreeBSD 10.3, leading to a false-positive
of the empty case.
Signed-off-by: Eric Wong <e@80x24.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>