bug#32773: clang: missing default include paths for C++

[fis trivial]

Tim Gesthuizen writes:

> On 19.09.2018 20:33, Pjotr Prins wrote:
>> Hi Tim,
>>
>> I am not sure this helps but in a project I have I use
>>
>>   CPPFLAGS= -std=c++11
>>
>> and
>>
>>   CPPFLAGS += -I$(GUIX)/include/c++ 
>> -I$(GUIX)/include/c++/x86_64-unknown-linux-gnu
>>
>> to find include files in Guix context with clang. Where $(GUIX) is the
>> profile.
>>
>> Similar to yours. Glad to hear of a better way.
>
> Yes, that seems to be quite the same to my workaround.
> Its just that this is  a workaround that is difficult to get working in
> some contexts: I want to use emacs-irony-mode through guix which is not
> really useable because it won't autocomplete any std::* stuff.
> If you take all packages that might want to use libclang and other
> features of clang it might be a better solution to find a proper fix for
> this problem. Also both workarounds need a user profile that is
> cluttered with all include files.
> I had a quick look into clangs source code how C_INCLUDE_DIRS is
> implemented. It should be more or less easy to add the same option for
> C++ (even C_INCLUDE_DIRS seems to be tinkered in to me).
> I just wanted to file a bug about this because fixing this is not
> trivial and I am not sure whether I will find the time right away to fix it.
> Fixing it would also have the benefit that I could send the patch to the
> LLVM mailing list and we might see the change upstream in the next LLVM
> version.
>
> Tim.

Just put everything into `C_INCLUDE_DIRS' should make it work, see:

https://github.com/trivialfis/guixpkgs/blob/c8a6871d2757557581640d7a14b4c9167459cb14/llvm.scm#L100

Using clang/clang++ from above link to compile a single translation unit should
work.  And there is `cquery' in `code.scm' within that repository, which relies
on clang to work, you can try it out.

It shouldn't take long to make LLVM and Clang in there ready to be merge in
core-update or staging (I don't know yet), it's just I don't feel comfortable
with the `clang-from-llvm' in guix, maybe someone could help carrying it out.

Jiaming