Add configure flags to for 32-bit build on 64-bit Mac OS X machine

I tried and found that configure fails on 64-bit Mac OS X machine with no configure flags.
Following https://github.com/Aries85/LightZone/commit/4e376e, I added "-m32" to CFLAGS and LDFLAGS and it builds successfully.

Could you review this pull request and merge it?
Thanks!
```js
var flags;

if (process.platform === 'darwin' && process.arch === 'x64') {
    flags = 'CFLAGS="-m32" LDFLAGS="-m32"';
}
```