1. Improvements to checking SWI-Prolog version requirements.
2. Added predicates for type checking.
3. Simplified way of identifying Apple/Mac OS.

* [FIX] Added the SWI-Prolog version check to the PGC startup procedure (regression).
* [OS_EXT,SC] Renamed `mac` to `apple`.
* [OS_EXT,SC] is_apple/0 now uses current_prolog_flag/2.
* [OS_EXT,SC] Removed set_os_flag/0 which is no longer needed now that `current_prolog_flag(apple,Value:boolean)` is added to swipl.
* [SWIPL_EXT,NEW] Added check_prolog_version/[1,3] for allowing projects that include the PGC to specify their own SWI-Prolog version requirement.
* [SWIPL_EXT,EXT] The version components may be uninstantiated (default value is zero).
* [SWIPL_EXT,EXT] Support for SWI-Prolog versions < 2.7.10.
* [TYPECHECK,NEW] Added several predicates for checking types, in line with swipl's must_be/2.