@MartinK: Actually, I didn't use anything CSSU thumb specific, especially it should work also with CSSU testing (but needs thumb compatible kernel) if I'm not mistaken. I decided to build against thumb because someone (in this thread?) recommended the linearo-toolchain for other reasons.

@marmistrz: Sorry, didn't see that because I installed libxc11 long before the thumb version of libx11 became available. The problem with libxc11 is that the libx11-data-version must match and I didn't foresee the introduction of a thumbed libx11. However I have installed libx11 thumb later without any problems (but not libx11-data-thumb, libx11-data has status "kept-back"). There is actually no need for a thumbed libx11-data package, because libx11-data contains no binaries.
Did you try apt-get install libx11-data=2:1.1.99.6+0m5?