Smoother libclang user experience

These patches provide the following improvements

- Remove the gcc/clang auto user options. We now use clang correctly, such that  this hack is not needed, but the system library paths are automatically found and added by clang. This should avoid the bug reports that where caused by adding incorrect system paths.

- Provide a proper error message, if libclang is explicitly requested, but it can not be loaded.

- Check if libclang can find its builtin include paths. If it does not, we try to find them for libclang. If we can also not find them, we warn the user about possibly slow code completion. This hopefully solves/avoids a lot of the bug requests we currently get about slow code completion.

- Silently check for libclang and use it if it is available and works. This will make libclang performance and features available, as soon as libclang.so is installed on the system. No further configuration is required.