fix: use build-time AHFAIL_INSTALL_DIR on macOS, not hardcoded /usr/local/lib

Apple Silicon Homebrew uses /opt/homebrew as prefix, so hardcoding
/usr/local/lib/ahfail/ahfail-display as DEFAULT_PATH breaks there.
build.rs now falls back to /usr/local/lib on macOS and /usr/lib on Linux
when AHFAIL_LIBDIR is not set; macOS uses the same concat!(env!(...))
approach as Linux.

install-macos.sh passes -Dlibdir so the baked-in path matches INSTALL_DIR.

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

scripts/install-macos.sh

@@ -34,10 +34,11 @@ PROJECT_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"
 cd "$PROJECT_DIR"
 
 step "Building ahfail..."
+MESON_FLAGS=("--buildtype=release" "-Dlibdir=${INSTALL_DIR%/ahfail}")
 if [[ -d builddir ]]; then
-    meson setup builddir --wipe
+    meson setup builddir "${MESON_FLAGS[@]}" --wipe
 else
-    meson setup builddir
+    meson setup builddir "${MESON_FLAGS[@]}"
 fi
 meson compile -C builddir