fix compilation against opencascade on linux targets

It turns out that I need some define magic in order to compile against
opencascade 6.7.1 on a Linux target:

@luca-heltai: Prior to inclusion can you please verify that with this
changes, opencascade still compiles and works fine on your setup? Ideally
before 12:00 (UTC+1) :-]
Ugh, this is ugly. Don't you have to do
```
  #ifdef HAVE_SYS_TYPES_H
  #  undef HAVE_SYS_TYPES_H
  #endif
```
to avoid preprocessor warnings if the flag wasn't set?