That's a good point, I believe you could run with generation off if you don't have SSE2.
How about add to the top of cryptopp/config.h:
#if !defined(_M_X64) && !defined(__x86_64__)
#define CRYPTOPP_DISABLE_SSE2 1
#endif
that would disable SSE2 for 32-bit builds. (at least with GCC or MSVC)