Silent C++ runtime dependence

[MathiasMagnus]

On Linux (but possibly Windows as well) when consuming glslang as a static library - as is the default with Vcpkg (which is how I found this bug) - in a C application, the glslang::glslang target silently depends on libstdc++.

If the project exports a C API, it likely wants to link the C++ runtime statically into itself when it is built as a static library. See WHOLE_ARCHIVE in the CMake docs.