This change fixes the build script's logic for resolving library
paths into linker flags when llvm-config returns the path to
a static archive; apparently some configurations will behave
this way.

Fixes #75.

There's work upstream on enabling this, so point to the upstream
issue to ease tracking.

Since the API changes are based on LLVM 21.1.0-rc1, this is currently
tagged as a prerelase (-rc1) crate.

Update for LLVM 21

See merge request !55