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) :-]
You are right. I thought I remembered that there is something in `-Wall` or `-Wextra` that warned about #undefing something that wasn't previously defined, but that's apparently not true.