cmake: set the project language

By default, CMake sets C and C++ as languages used by the project.
This makes the configuration failed if no C++ compiler is found.

Since libwesockets in written in pure C, no C++, there is no reason to
disable it when the toolchain has no C++ support.

This patch just make the CMake code reflecting the real status of the
project with regard to the languages used.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>