Fix CMake for OSX

Fix glitches that prevent the CMake build from working on OSX:
— Must explicitly set -target flag for clang for -m64 flag to work (as
is done in Makefile)
— Remove unnecessary -ltinfo directive (doesn’t exist on OSX)
Query llvm for system libs to use on os x and linux
cmake is the devil
Merge pull request #500 from halide/cmake

Fix CMake for OSX