CMakeLists.txt: do not check for CXX
CMakeLists.txt: do not check for CXX

The CXX compiler is not needed, force to check only for C.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

Conan-Kudo/librepo,Conan-Kudo/librepo,rpm-software-management/librepo,rholy/librepo,rholy/librepo,bgamari/librepo,Conan-Kudo/librepo,bgamari/librepo,bgamari/librepo,rholy/librepo,Tojaj/librepo,Tojaj/librepo,rpm-software-management/librepo,rpm-software-management/librepo,Tojaj/librepo,rholy/librepo