Fix error locating llvm-ar during build on Linux

[ddevault]

This explicitly sets the ar to llvm-ar when running cmake. Prior to this
commit, CMake was told that the prefix for all tools is "llvm-", but
this did not affect ar. Explicitly setting CMAKE_AR to "llvm-ar" is a
bit of a workaround considering the rest of the tools are set
automatically, but I cannot think of any negative side effects of doing
so.

This fixes #33.

[ddevault]

A note: _CMAKE_TOOLCHAIN_PREFIX is an undocumented internal variable that CMake uses. It may be a better choice to simply explicitly declare the entire toolchain.

[janvorli]

"which llvm-ar" doesn't show anything on my Ubuntu 14.04 with LLVM 3.5 installed. There is no llvm-ar on the system, just llvm-ar-3.5.
I've tried your change and it breaks the build on my Ubuntu due to this.

[ddevault]

Hmm, it's odd that your llvm-ar executable would be named that. That sounds like a packaging error. Let me see if I can determine a consistent way to locate that file. Are the rest of your llvm tools (i.e. llvm-as, llvm-link, etc) postfixed with a version number?

[ddevault]

Looks like they're packaged without the postfix: http://packages.ubuntu.com/trusty/amd64/llvm/filelist

This is pretty weird.

[janvorli]

Yes, all of them have the suffix except of the llvm-clang. The package we install on Linux is called llvm-3.5

[ddevault]

Thanks for clarifying. That package is a pretty non-standard install of llvm, so some thought will have to be put into how to support it. I'm spinning up an Ubuntu VM now to see what I can do.

[janvorli]

So the package that you have installed is called just llvm? That one on Ubuntu contains an old version (3.4.0-ubuntu1).

[ddevault]

I'm testing it on Arch Linux, which uses llvm 3.5 in the llvm package. The typical llvm install does not include a postfix.

[ddevault]

@janvorli I have just force pushed an alternative fix. Would you be so kind as to test it under Ubuntu for me?

[ddevault]

Build server seems to have gotten through the cmake configuration part of this ordeal, so presumably this is good to go.

[janvorli]

It works fine on my Ubuntu and the change looks good to me. Thank you very much for fixing the issue!

[ddevault]

Happy to help 😄

[janvorli]

@sergeyk @jkotas Could you please take a look as well?

[jkotas]

LGTM