Improve clang detection based on #48 for tests

[daniel-j]

Should add this proc to the module so that futhark can use it too, as described in #48

[PMunch]

Well with the ability to pass sysPaths as a define I don't think it's necessary to add it to Futhark. Libraries using Futhark should simply add this to their config.nims file. I guess we could have a futhark.nims file that can be imported into config.nims so that getClangIncludePath could be used without having to define it every time though.

[daniel-j]

Readme should be updated to mention it in that case. I was thinking something along this (although my futhark dot here is not needed, the proc would just be exported and usable):

import futhark
importc:
  sysPath futhark.getClangIncludePath()
  ...

[PMunch]

Hmm, that's fair. Should work just as well. Maybe it should even default to that if no sysPath was defined..