Fix fat builds on pre-Leopard and post-Lion versions of OS X.

Prior to Tiger, only 32-bit PowerPC was supported, so don't do fat
builds.  For 10.4 through 10.4.3, only 32-bit and 64-bit PPC were
supported; for 10.4.4 through 10.4.6, 32-bit x86 was also supported; for
all later versions of Tiger, 32-bit and 64-bit x86 were also supported.

For everything after Snow Leopard, not just for Lion, only 32-bit and
64-bit x86 are supported; don't just check for Lion, assume anything for
which we don't have specific cases is like that (unless and until Apple
drops support for 32-bit x86 applications).