build: Fix libraries for HP-UX.

* configure.ac: HP-UX requires -lpthread and -lrt.

Since HP-UX has stub for pthread functions in C library, it
is required to link -lpthread. POSIX semaphore functions like
sem_init is required to link -lrt. Tested on ia64-hp-hpux11.31.

• Reported-by: John Hirsch
• GnuPG-bug-id: T3980
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>