Consider including a pkg-config file.

[dabrahams]

Right now, SPM clients need to create their own, but we /could/ ship one in the package. We'd just steal the generation script from Swifty-LLVM and run that… except that I think we're getting absolute paths from llvm-config on Windows, and that could be a problem because people will decompress their LLVM packages into arbitrary locations. Something to look into.

[tothambrus11]

We could investigate if pkg-config supports relative paths. We can take two absolute paths on Windows and construct a relative path between them.

[tothambrus11]

https://manpages.debian.org/stretch/pkg-config/pkg-config.1.en.html#:~:text=it%20can%20also%20be%20used%20to%20make%20.pc%20files%20relocatable

pcfiledir
The installed location of the .pc file. This can be used to query the location of the .pc file for a particular module, but it can also be used to make .pc files relocatable. For instance:

prefix=${pcfiledir}/../..
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

PKG_CONFIG_PATH
A colon-separated (on Windows, semicolon-separated) list of directories to search for .pc files. The default directory will always be searched after searching the path; the default is libdir/pkgconfig:datadir /pkgconfig where libdir is the libdir for pkg-config and datadir is the datadir for pkg-config when it was installed.

Using these two pieces of information, we could generate a relocatable .pc file that we can distribute together with the library. Then the docker runner needs to update the environment variable with the PKG_CONFIG_PATH environment variable with the path of wherever we unpacked LLVM.

Note that we would need to replace all the absolute paths returned by llvm-config by a concatenation of pcfiledir and a relative suffix from there.

Now the only remaining issue is related to finding libzstd. Maybe we can solve that separately from this, and just whenever we rely on the llvm package in swift, we could also add the libzst as a library that is found by pkgconfig. See #8 and https://github.com/hylo-lang/hylo/blob/main/.devcontainer/make-pkgconfig.sh