[PATCH] clang-cl: Add support for -o

[Stephen Kelly]

Alp Toker wrote:

>> To self-host, I currently generate a build.ninja file with CMake and
>> replace the path to cl.exe with a script.  =(  I've been holding off
>> on documenting self-hosting with clang-cl until that gets fixed.
> 
> Reid, do you think we'll be OK if we can instead find a configure switch
> that forces CMake to believe that we're MSVC for the time being?
> Meanwhile we can work with CMake upstream to resolve detection.

This seems related:

 http://public.kitware.com/Bug/view.php?id=14863

CMake just does not know enough about clang-cl and how to invoke it. So far 
CMake support for clang-cl requires the VS toolchain. 

 http://public.kitware.com/Bug/view.php?id=14458

It probably needs to get a new (set of) files in Modules/Platform or 
Modules/Compiler. If such files can be written, you can host them elsewhere 
until they are in cmake upstream (and you depend on the version they ship 
with).

> I worry that external tools and projects will start depending on hybrid
> clang-cl.exe options and we'll be stuck with this quirky in-between mode.

Indeed. I recommend teaching cmake what you need it to know instead.

Thanks,

Steve.