Issues building host tools together with a target build

[barche]

When building tools that have to run on the host system (e.g. wayland-scanner in Wayland_jll) things don't always work smoothly:

• Host pkgconfig prefixes all paths with a superfluous /workspace/destdir Fixed already
• Target libraries end up in the host sysroot, at least on freeBSD
  Possibly these are bugs somewhere in the sysimages.

Originally posted by @barche in #10876 (comment)

[giordano]

Host pkgconfig prefixes all paths with a superfluous /workspace/destdir

Isn't that fixed by JuliaPackaging/BinaryBuilder.jl#1346 already? Of course needs a package built after that PR.

[barche]

Yes, indeed!