Detect python 3 and abort during build

[brson]

LLVM cannot build with python 3 but tries anyway. This has caused confusion a number of times.

[graydon]

This was fixed by the pull req. Closing.

[bstrie]

Turns out this didn't actually fix the problem. At the time I had (wrongly) assumed that the Python version detected during Rust's configure phase was being somehow communicated to LLVM. Looking a bit more closely, I'm not sure that Rust does anything at all with the detected version of Python; perhaps an artifact of an era when Rust had a Python dependency?

bbrittain (who rediscovered this bug) and I discussed some hacky workarounds and finally decided that the best thing to do is to send a patch upstream to LLVM (which, despite having a hard dependency on Python 2, never bothers to check for Python 3 and just fails with cryptic errors). Once the fix is accepted we can integrate it into Rust's fork of LLVM.

[benbrittain]

(I'm bbrittain)
LLVM assumes, and has hardcoded, that python means a python2.x. Bleeding-edge linux distros, such as arch, now have 'python' mean python3. It's easy to symlink around, but it really would be better to just fix it.

[brson]

Reopening. @cavedweller you have a pull request on brson/llvm that I completely forgot about. Is it still appropriate to merge? Do you know if llvm has the fix upstream yet?

Sorry I lost your patch. Feel free to contact me privately if that happens again.

[brson]

This does continue to come up. Would be nice to fix definitively.

[benbrittain]

LLVM has not fixed it upstream.
frankly, I couldn't find a way to submit the bug fix due to them using SVN...

[brson]

erickt has rebased us onto LLVM HEAD and I rebased your patch on top of that. Hopefully this will get in today.

[benbrittain]

sweet, thanks!

[brson]

The fix is on incoming.