macOS build.sh: Blacklist linkat and unlinkat

Those are only available starting macOS 10.10.

extras/package/macosx/build.sh

@@ -204,6 +204,8 @@ export ac_cv_func_fdopendir=no
 export ac_cv_func_openat=no
 export ac_cv_func_fstatat=no
 export ac_cv_func_readlinkat=no
+export ac_cv_func_linkat=no
+export ac_cv_func_unlinkat=no
 
 # Added symbols between 10.7 and 10.9
 export ac_cv_func_memset_s=no