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) :-]
No, as far as I recall, #undef ining an undefined macro is perfectly valid. At least no gcc version (from 3.4 - 4.9) and no clang version complains about.