fix(full+build): unblock glib/mesa/libinput; close fork-staleness gap

- build-redbear.sh: detect changes to the 4 forks the staleness loop missed
  (userutils pkgar; syscall/libredox/redox-scheme crate path-deps force a full
  userspace relink) — prevents stale-binary skew (bash -n validated).
- glib: drop the stale '#undef AT_FDCWD' Redox workaround — relibc now provides
  faccessat()/AT_FDCWD, and glib's HAVE_FACCESSAT path needs AT_FDCWD.
- mesa: remove the dropped '-Dosmesa=true' meson option (unknown option) and
  guard the osmesa.pc pkg-config edit.
- libinput: unblocked by syncing the fresh relibc libc.a (with timerfd_*) into
  the stale prefix sysroot (prefix-sysroot staleness — see notes).

local/scripts/build-redbear.sh

@@ -623,6 +623,34 @@ if [ "$NO_CACHE" != "1" ]; then
         fi
     done
 
+    # Shared forks not covered by the pkgar loop above. userutils ships a pkgar
+    # (login/getty/etc); syscall/libredox/redox-scheme are Cargo path-dep crates
+    # with NO pkgar of their own but are linked into most of userspace, so
+    # cookbook's per-recipe cache cannot detect a change in them. Detect a source
+    # change and force a full userspace relink to prevent stale-binary skew.
+    for src in userutils syscall libredox redox-scheme; do
+        src_dir="$PROJECT_ROOT/local/sources/$src"
+        fingerprint="$PROJECT_ROOT/repo/x86_64-unknown-redox/$src.source-fingerprint"
+        [ -d "$src_dir/.git" ] || continue
+        src_commit=$(git -C "$src_dir" rev-parse HEAD 2>/dev/null || echo "")
+        last_commit=$(cat "$fingerprint" 2>/dev/null || echo "")
+        src_dirty=0
+        if ! git -C "$src_dir" diff --quiet HEAD 2>/dev/null \
+            || ! git -C "$src_dir" diff --cached --quiet HEAD 2>/dev/null \
+            || [ -n "$(git -C "$src_dir" ls-files --others --exclude-standard 2>/dev/null)" ]; then
+            src_dirty=1
+        fi
+        # Only act when a prior fingerprint exists (skip the first build, which
+        # cooks everything anyway).
+        if [ -n "$last_commit" ] && { [ "$src_commit" != "$last_commit" ] || [ "$src_dirty" = "1" ]; }; then
+            echo ">>> Stale shared fork $src detected; forcing full userspace relink (correctness)..."
+            STALE_DETECTED=1
+            USERSPACE_RUNTIME_STALE=1
+            [ "$src" = "userutils" ] && rm -f "$PROJECT_ROOT/repo/x86_64-unknown-redox/userutils".*
+        fi
+    done
+
+
     if [ "$USERSPACE_RUNTIME_STALE" = "1" ]; then
         echo ">>> Cleaning stale build/sysroot dirs (userspace-linked runtime changed)..."
         find "$PROJECT_ROOT/recipes" "$PROJECT_ROOT/local/recipes" \
@@ -867,7 +895,7 @@ for pkg in $PRECOOK_PKGS; do
     fi
 done
 
-for src in relibc kernel base bootloader installer; do
+for src in relibc kernel base bootloader installer userutils syscall libredox redox-scheme; do
     src_dir="$PROJECT_ROOT/local/sources/$src"
     if [ -d "$src_dir/.git" ]; then
         git -C "$src_dir" rev-parse HEAD 2>/dev/null > \