Use explicit version number for pkgconfig

[kofigumbs]

I believe that the llvm-config executable is typicaclly a link to the specific version, so it should be a no-op for the most part. However, I was having trouble building this on Ubuntu 16.04 since I only have the llvm-config-3.9 executable. This small change fixed it.

I cannot confirm this for macOS though.

Thanks

[CodaFi]

This may fail the Linux build because they are in the opposite situation where they have Clang 3.5.0 installed and a symlink pointing to that version of the tool. We're explicitly ln'ing the freshly installed LLVM Config in place for that reason.

[CodaFi]

Errr, or it may do the opposite and fail the macOS build because brew does The Right Thing ™ with llvm-config 😆

[harlanhaskins]

I think we can actually do better. We could have the script try llvm-config-3.9 and then llvm-config if that's not found.

[harlanhaskins]

I did a similar thing in the swift utils folder: https://github.com/apple/swift/blob/master/utils/swift_build_support/swift_build_support/toolchain.py

[CodaFi]

EDIT: Whoa, why did this wind up here?

An example of a diagnostic we should squash at some point

[harlanhaskins]

@hkgumbs: So yeah, this unfortunately won't work as-is on macOS. Thanks for bringing up the issue though! Would you want to implement the more complete beavior? 😃

[kofigumbs]

Thanks for the super quick replies! I'll work on implementing the more complete behavior and re-open this when it's ready