get_ppid_of, get_progname_of: Fix compilation error on Mac OS X < 10.5.
Reported by Ryan Schmidt <gnulib@ryandesign.com> in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00011.html>.
* lib/get_ppid_of.c: Include AvailabilityMacros.h
(get_ppid_of): Use MAC_OS_X_VERSION_MAX_ALLOWED and
MAC_OS_X_VERSION_MIN_REQUIRED.
* lib/get_progname_of: Include AvailabilityMacros.h
(get_progname_of): Use MAC_OS_X_VERSION_MAX_ALLOWED and
MAC_OS_X_VERSION_MIN_REQUIRED.