Driving a differential signal from FPGA
I found my answer on page 260/261 of this document. I have to use the OBUFDS primitive from the unisim library. My final code contains the lines:

library unisim;
use unisim.vcomponents.all;

differential_pair_driver : OBUFDS
port map (
    O =&gt; ck,
    OB =&gt; ck_n,
    I =&gt; clk
);


Edit: What I didn't understand when I wrote this answer, is that the OBUFDS is a so-called primitive. The primitives you can use can be different for another FPGA. I think that in general, the most appropriate way is to look up the primitives for your specific fpga. I have a Spartan-6. Googling 'spartan 6 primitives' yields this document as the first result. The OBUFDS can be found on page 195.