Build fatbinary

Currently we're only building (it seems) the x64 binary for the mac.

You can set -DDEFAULT_SYSROOT=/path/to/MacOSX11.1.sdk at build time or do export SDKROOT=/path/to/MacOSX11.1.sdk at runtime.

You need to compile with clang -arch arm64 -arch x86_64 to get a fat binary out of clang. You need to do this for Apple clang as well.
