[mac] Conditionalize the inclusion of "AvailabilityMacros.h".

The native SDK on earliest Mac OS X (10.0-10.1) did not have
"AvailabilityMacros.h".  To prevent the inclusion of missing
header file, ECANCELED (introduced in 10.2) in POSIX header
file <errno.h> is checked to detect the system version.

* include/freetype/config/ftconfig.h: Conditionalize the
inclusion of "AvailabilityMacros.h".
* builds/unix/ftconfig.in: Ditto.
* builds/vms/ftconfig.h: Ditto.
4 files changed