[CMake] How to setup toolchain correctly for C compiler only (no CXX present for target)

[Martin Kupke]

Is there a way to setup the toolchain file in a way to work only with a 
C compiler?
What if no C++ compiler is present (or wanted) for the target HW?
E.g. we use standard C compiler for different processors on our embedded 
solutions. We need to buy the compiler for C and / or C++, but mostly we 
only develop in C and therewith we don't buy the C++ licence. While 
configuring my build environment with CMake the first time the 
CMakeTestCCompiler and even the CMakeTestCXXCompiler used.
I wasn't able to suppress the CXX part. Is there any option to do this?