fix(packaging/gamescope): the shipped compositor starts on SteamOS — a rolling-distro libstdc++ never followed it there

`punktfunk-gamescope-3.16.25.pfhdr2-1` off the pacman repo cannot start on SteamOS 3.8.16:
`/usr/lib/libstdc++.so.6: version 'GLIBCXX_3.4.35' not found`. The Arch container CI builds it in
is on gcc 16.1.1; SteamOS ships libstdc++ 3.4.34 and moves when Valve says so. Nothing else about
the binary was wrong — every other soname resolved on the box, and glibc was never close (it asks
for 2.38 at most against SteamOS's 2.41) — so the one dynamic C++ runtime was the whole reason the
gamescope backend's own most important platform got a package that dies at `--version`.

The C++ runtime therefore goes static, for the same reason wlroots already does: this binary is
built on a rolling distro and has to start on a frozen one. It is safe here because gamescope
links no shared C++ library at all — its NEEDED list is all C, and glslang/SPIRV are build-time
only — so no C++ ABI crosses a shared boundary. Cost is ~1 MB (5.9 → 7.1). The flags are appended
to LDFLAGS rather than passed as `-Dcpp_link_args`, which would replace the value meson derives
from the environment and silently drop makepkg's `-z relro`/`-z now`/`--as-needed`.

A static runtime is invisible in a passing build and only surfaces as a binary that will not start
somewhere else, so the build now asserts it: no `libstdc++` in NEEDED, which needs no version
threshold to check and turns the regression back into a build failure.

Verified by building in `archlinux:base-devel` — the environment arch.yml uses, gcc 16.1.1 and
glibc 2.44, both far newer than the target — and running the result on SteamOS 3.8.16 with nothing
supplied: banner `punktfunk-gamescope version 3.16.25-4-g6bbe157+pfhdr2`, no libstdc++ in NEEDED,
max GLIBC_2.38, every soname resolving. The host's whole HDR gate chain then answers on SteamOS
for the first time (780M / RADV PHOENIX): 10-bit PQ capture offered, cursor painted in-node,
native-plane HDR and GameStream HDR capable both true once `PUNKTFUNK_GAMESCOPE_HDR` is on, false
with the distro's gamescope.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packaging/gamescope/build-punktfunk-gamescope.sh

@@ -95,6 +95,19 @@ echo "==> configuring"
 # there would not start on the host. Pinning the fallback makes the outcome the same everywhere.
 # (gamescope's own meson.build hard-errors if libliftoff/vkroots are missing from this list, so
 # all three go together.)
+#
+# The C++ runtime goes STATIC for the same reason wlroots does: this binary is built on a ROLLING
+# distro and has to start on a FROZEN one. Arch's gcc (16.1.1 when this was written) makes the
+# compositor require `GLIBCXX_3.4.35`, and SteamOS 3.8.16 ships libstdc++ 3.4.34 — so the published
+# Arch package died on the very platform the gamescope backend matters most on ("version
+# GLIBCXX_3.4.35 not found"), while every other soname resolved and glibc was never close to the
+# limit (the binary asks for 2.38 at most; SteamOS has 2.41). Safe because
+# gamescope links NO shared C++ library (its `NEEDED` list is all C — glslang/SPIRV are build-time
+# only), so no C++ ABI ever crosses a shared boundary; it costs ~1 MB.
+# Appended to LDFLAGS rather than passed as `-Dcpp_link_args`, because that option would REPLACE
+# the value meson derives from the environment and silently drop makepkg's hardening flags
+# (`-z relro`, `-z now`, `--as-needed`).
+export LDFLAGS="${LDFLAGS:-} -static-libstdc++ -static-libgcc"
 meson setup "$BUILD" "$SRCDIR" \
   --prefix="$PREFIX" \
   --buildtype=release \
@@ -112,6 +125,17 @@ ninja -C "$BUILD" ${JOBS:+-j "$JOBS"}
 # distro's gamescope package — and we need none of them: the host only ever execs the compositor.
 BIN="$BUILD/src/gamescope"
 [ -x "$BIN" ] || { echo "build produced no $BIN" >&2; exit 1; }
+# The static C++ runtime above is invisible in a successful build and only shows up as a binary
+# that will not start on an older distro — so assert it here, where a mistake is a build failure
+# instead of a package that dies at `--version` on SteamOS. No `libstdc++.so.6` in NEEDED is the
+# whole invariant (and it needs no version threshold to check).
+if command -v objdump >/dev/null; then
+  objdump -p "$BIN" 2>/dev/null | grep -q 'NEEDED.*libstdc++' && {
+    echo "built binary links libstdc++ dynamically — the static C++ runtime did not take, and this" >&2
+    echo "package would not start on a distro older than the build host" >&2
+    exit 1
+  }
+fi
 DEST="${DESTDIR}${PREFIX}/bin/punktfunk-gamescope"
 echo "==> installing $DEST"
 install -Dm755 "$BIN" "$DEST"