How do I build a static library for iPhone?

I think that I've looked everywhere for an answer to my problem but without any luck.

I'm trying to create a simple static lib to run on the iPhone device but I keep ending up with XCode saying that "file is not of required architecture" and I've tried every build flag that I found without any luck.

I've got it to work on the emulator building it like this:

$gcc-4.2 -c StaticHelloWorld.c -o StaticHelloWorld.o
$ar rcs libstatichelloworld.a StaticHelloWorld.o

But how do I build it for device?