Add pthread-related tests.

Add pthread-related tests.

* test-suite/standalone/Makefile.am (test_scm_spawn_thread_CFLAGS,
  test_scm_spawn_thread_LDADD, test_pthread_create_CFLAGS,
  test_pthread_create_LDADD, test_pthread_create_secondary_CFLAGS,
  test_pthread_create_secondary_LDADD): New variables.
  (check_PROGRAMS)[BUILD_PTHREAD_SUPPORT]: Add `test-scm-spawn-thread',
  `test-pthread_create', `test-pthread_create-secondary'.
  (TESTS)[BUILD_PTHREAD_SUPPORT]: Likewise.

* test-suite/standalone/test-scm-spawn-thread.c,
  test-suite/standalone/test-pthread-create.c,
  test-suite/standalone/test-pthread-create-secondary.c: New files.