posix-lock: Support GNU C library 2.34 or later.
* configure.ac: Add check for sys/single_threaded.h. * src/posix-lock.c [USE_POSIX_THREADS_FROM_LIBC]: Use __libc_single_threaded to detect if it's single threaded.
- GnuPG-bug-id: T5547
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>