[CMake] Portable deactivation of C++ check

[Eduard Bloch]

Hello,

I stumbled over a problem with the current CMake. It just fails if no
CXX compiler is installed! Sometimes I would like not to depend on a C++
compiler because it is just not used.

What is the portable solution to work around that? A simple one may be
setting -DCMAKE_CXX_COMPILER=/bin/true in the initial call of cmake, but
it requires a wrapper to pass that value, and it is hardly portable.

Regards,
Eduard.