Fix memalign darwin

Fix to have the CMake build do what the GNU Autotools build system did for memory alignment, that somehow was not included in the conversion to CMake. This change impacts OSX only, and mostly just older OSX -- likely just 10.6 and older. Xcode 5 introduced posix_memalign, so this change does not impact 10.8 or newer, and likely 10.7 or newer (but I haven't tested). Small, simple changes.