get_ppid_of, get_progname_of: Fix runtime error on Mac OS X < 10.5.
Reported by Evan Miller <emmiller@gmail.com>
<https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00081.html>.
* lib/get_ppid_of.c (proc_pidinfo): Declare with WEAK_IMPORT_ATTRIBUTE.
(get_ppid_of): Test whether the symbol declared weak evaluates to
non-NULL before invoking it.
* lib/get_progname_of (get_progname_of): Declare with
WEAK_IMPORT_ATTRIBUTE.
(get_ppid_of): Test whether the symbol declared weak evaluates to
non-NULL before invoking it.