Blender Git Loki

CMake: Default to C11 for Clang and GCC

The option WITH_C11 is gone, and C++ is defaulting to C++11 now,
so guess it's fine to assume we need C11 now.

This is technically what we use anyway, with all the re-definitions
of structs (like when we typedef anonymous struct in a header file
first, and them define it to a proper structure in implementation
file).