Do not provide the clang/gcc autoloader any more

The clang and gcc autoloaders are not necessary any more to optain the
system include paths. Due to the previous commit, the include paths are
now correctly added by the clang driver. libclang always used internally
the clang driver and never had problems finding the system include paths.

We remove these options as incorrectly derived include paths can cause
all kind of spurious problems.

We now print the following message, if someone manually adds 'clang' or
'gcc' to the auto_user_options:

  'gcc' in clang_auto_user_options is deprecated.
Always initialize g:clang_library_path

Like this we can avoid special cases in the later code path
Make it clear that we disable libclang if the initialization fails
Catch Exception when loading libclang fails

Instead of passing on the exception, we now provide a proper error
message to the user and fall back to the clang executable.
libclang: Rework handling of builtin includes

If libclang is installed in some non-default location, it sometimes
fails to find the path to the builtin includes. This will cause an error
when including these headers which again blocks precompiled header
caching. Even though autocompletion often still works, it is annoyingly
slow.

We now check explicitly if the builtin includes are available. If they
are not we try to find them ourselves. Only if this does not work, we
warn the user and ask him to report a problem.
Automatically use libclang if available

If the user did not request anything specific, we first try to use
libclang and only fall back to the clang executable if libclang could
not be found. Error messages are only printed in debugging mode or
if the user explicitly requested libclang.

This patch allows us to use libclang.so as soon as it can be found in
the system library path. No further configuration is required in
.vimrc.