pkg-config-wrapper: mangle PKG_CONFIG_PATH{,_FOR_BUILD} correctly

[sternenseemann]

Previously, mangleVarList would be used which would concatenate the
variables using a space as a separator. Paths are however separated by
: in PKG_CONFIG_PATH leading to entries being broken.

This is fixed by introducing mangleVarListGeneric which allows us to
specify the desired separator.

Reproducer for the issue prior to this change:

$ nix-shell -A pkgsLLVM.wayland
[nix-shell] $ pkg-config --libs expat
Package expat was not found in the pkg-config search path.
Perhaps you should add the directory containing `expat.pc'
to the PKG_CONFIG_PATH environment variable
No package 'expat' found
$ printf 'Host: %s\nBuild: %s' $PKG_CONFIG_PATH $PKG_CONFIG_PATH_FOR_BUILD
Host: /nix/store/5h308a4ab8w7prcp8iflh5pnl78mayi2-expat-2.2.10-x86_64-unknown-linux-gnu-dev/lib/pkgconfig:/nix/store/z3y9ska2h4l1map25m195iq577g7g3gz-libxml2-x86_64-unknown-linux-gnu-2.9.12-dev/lib/pkgconfig:/nix/store/lbz5m1s0r7zn0cxvl21czfspli6ribzb-zlib-1.2.11-x86_64-unknown-linux-gnu-dev/lib/pkgconfig:/nix/store/rfhvp8r8n3ygpzh8j0l34lk8hwwi3z0h-libffi-3.3-x86_64-unknown-linux-gnu-dev/lib/pkgconfig
Build: /nix/store/dw11ywy7qwfz53qisz0dggbgix88jah2-wayland-1.19.0-bin/lib/pkgconfig

strace reveals the issue:

stat("/nix/store/dw11ywy7qwfz53qisz0dggbgix88jah2-wayland-1.19.0-bin/lib/pkgconfig /nix/store/5h308a4ab8w7prcp8iflh5pnl78mayi2-expat-2.2.10-x86_64-unknown-linux-gnu-dev/lib/pkgconfig/expat-uninstalled.pc", 0x7fff49829fa0) = -1 ENOENT (No such file or directory)

In the pkg-config wrapper $PKG_CONFIG_PATH_FOR_BUILD and
$PKG_CONFIG_PATH are concatenated with a space which leads to two paths
being messed up. This issue likely only affects native cross
compilation.

---

nix-build -A pkgsLLVM.wayland
/nix/store/sa1qfq4s9vq5cif8y5vdfsa3gl29byk1-wayland-x86_64-unknown-linux-gnu-1.19.0

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Ericson2314]

Woah! I could hardly believe this was problem so long without being caught. Sure enough on 21.09 it's a problem, but zlib is the last package in both lists and only graphviz can't be looked up, and perhaps with the -I & -L injection we do that wasn't a problem.