[mac os x] Need to set x86_64 on snow leopard with macports

When compiling orocos 1.10 on mac os x SL with macports, I had to
manually set CMAKE_OSX_ARCHITECTURES to x86_64 to be able to compile
orocos. Although the/my ports libraries are compiled in 64 bit mode,
gcc apparently generates i386 code by default (have 32 bit kernel
running).

WORKSFORME...

k