Now consider your code:-

if (nummer[i] < 0 && nummer[i] > 9){


Now according to me since this is a char array, you numbers will be stored as chars and hence you will need to check for their...

Ok got a solution to your problem here (https://answers.launchpad.net/ubuntu/+question/764) with just a little googling.

It says to install the libc6-dev package.

Well can you tell us what you are trying to achieve here actually, I don't have much to go on from what you have posted !!
crt1.o looks to be an object file and it seems like your linker (ld) can't...