Add compile-time flags for mise python-build on NixOS

python-build compiles from source and needs headers/library paths.
nix-ld only handles runtime linking for prebuilt binaries. Set
CFLAGS, LDFLAGS, and PKG_CONFIG_PATH via sessionVariables so
configure scripts find zlib, openssl, readline, etc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/ringtail/configuration.nix

@@ -1,5 +1,9 @@
-{ config, pkgs, ... }:
+{ config, pkgs, lib, ... }:
 
+let
+  # Libraries needed by mise-compiled runtimes (python-build, etc.)
+  buildDeps = with pkgs; [ zlib readline bzip2 xz libffi ncurses sqlite openssl ];
+in
 {
   # Allow unfree packages (NVIDIA drivers, Steam)
   nixpkgs.config.allowUnfree = true;
@@ -138,17 +142,14 @@
 
   # Allow running dynamically linked binaries (mise-installed runtimes, etc.)
   programs.nix-ld.enable = true;
-  programs.nix-ld.libraries = with pkgs; [
-    icu
-    openssl
-    zlib
-    readline
-    bzip2
-    xz
-    libffi
-    ncurses
-    sqlite
-  ];
+  programs.nix-ld.libraries = buildDeps ++ [ pkgs.icu ];
+
+  # Compile-time flags for mise python-build and similar source builds
+  environment.sessionVariables = {
+    PKG_CONFIG_PATH = lib.makeSearchPath "lib/pkgconfig" (map lib.getDev buildDeps);
+    CFLAGS = lib.concatMapStringsSep " " (p: "-I${lib.getDev p}/include") buildDeps;
+    LDFLAGS = lib.concatMapStringsSep " " (p: "-L${lib.getLib p}/lib") buildDeps;
+  };
 
   # Fonts
   fonts.packages = with pkgs; [