Fix build without threads

* src/gen-posix-lock-obj.c: properly guard inclusioin of pthread.h
* tests/t-lock.c: likewise
* tests/t-poll.c: likewise

Although ./configure checks for thread support, gen-posix-lock-obj and
two tests still include pthread.h unconditionally.

Guard that inclusion using the same condition as all other uses of
pthread-related code.

• Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>