Add x86_64-darwin support.
Update pre-processor definitions for x86_64-darwin.

Reflects Xcode 5.0.2 w/ clang 5.0 on 10.9.1 (Mavericks).  I removed
everything that wasn’t explicitly present in the output of “clang -dM
-E -x c /dev/null” and added enough markers to bind it to 64-bit Darwin
w/ clang.
Restore blank definitions.
Remove __BLOCKS__ since we can't parse it yet.
Merge pull request #14 from ddp/x86_64-darwin

Add x86_64-darwin support.