build: Add support for IBM z/OS, fixing -lpthread check with glibc.

* configure.ac: Care about PRINTABLE_OS_NAME for IBM z/OS.
Handle have_pthread for IBM z/OS.
No check with -lpthread when we know about have_pthread.

The variable have_pthread in configure script is whether or not the
pthread API is available on the host. Linking is handled with
libgpg-error using GPG_ERROR_MT_LIBS.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>