Hi Christian,

> When `--without-clang` or `--without-llvm` are passed to configure,
> this seems to be interpreted as "with" instead of "without". Omitting
> each disables them as per defaults.

Ooops!

> I'm not an autoconf pro, but I think there is some inconsistency
> between with_clang / with_clang_plugin and with_llvm /
> with_llvm_plugin and maybe their default values in the configure
> script.

Agreed.

Please could you try out the attached patch and let me know if it
works for you ?

It should harmonize the configure options, so that by default
everything is built and that in order to disable a component you
need to add --without-<name> to the command line.  Plus the names
for the plugin options are now consistent, ie --without-gcc-plugin
--without-clang-plugin and --without-llvm-plugin.

Cheers
   Nick