Define `PLATFORM_MacOSX`
The cdef.h header is used to automatically determine how a symbol should be defined. Since no platform was defined, the `__DARWIN_ONLY_UNIX_CONFORMANCE` would not properly get set to 1 if the arch is not `i386`.
This fixes the issues where x86_64 would have `$UNIX2003` appended to some of the symbols.
I recommend refering to the `COMPAT(5)` man page for a better understanding on this issue.