Clean up OpenVMS support.

This avoids cluttering the source code with VMS-specific changes.

* include/freetype/internal/ftcalc.h [__VMS], src/base/ftcalc.c
  [vms_auto64_source]: Remove.

* builds/vms/patch_ftcalc.sed: New file.

* vms_make.com: Change macro definitions while this script is running; we
  use `GNU sed` to inject the necessary preprocessor code on the fly.