fix(cmake): specify project to only use c

[kr-t]

I am not sure if this is a correct move, but I think it is.

While using a minimal docker image to cross-compile the project I work on (which uses libcoap), I have got a following error:

CMake Error at libcoap/CMakeLists.txt:13 (project):
  No CMAKE_CXX_COMPILER could be found.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

This error is raised, because the project requires CXX and C languages in Cmake. While I can workaround this without issues (by using dummy path to CXX and some additional Cmake arguments), I think the correct thing would be to remove dependency on CXX. Since cpp in the whole project is only used for zephyr test, it shouldn't affect anything. I have created before hand a fork and tested things in CI, and everything seems to work as before (why wouldn't it). Functionally, my cross-compiled application turned out fine, without needing CXX and libcoap worked as expected.

I would understand if you want to keep it as it is, but I think removing unnecessary dependencies are always a good solution. In worst case, once libcoap actually uses CXX (if ever), it can just add it back.

Feel free to close the PR without merging, if you disagree.