Add libXt-1.3.1 build

[jserv]

This build libXt-compat.so from pinned upstream libXt-1.3.1 against the SDL2-backed Xlib shim. Output exports 299 Xt symbols; depends only on libX11-compat.so and libSystem.

Fill the Xlib symbols libXt actually calls.
XAddConnectionWatch, XRemoveConnectionWatch, XInternalConnectionNumbers, XProcessInternalConnection: stubs, since the SDL backend has no internal connections to fold into the event loop.
XConvertCase: carved verbatim from libX11 KeyBind.c with attribution, since the surrounding KeyBind.c overlaps with src/input.c's SDL-backed keysym handling.
XrmQGetResource, XrmQGetSearchList, XrmQGetSearchResource: bridge the quark API onto the existing string-based database by encoding the database pointer plus the widget prefix arrays into the search list, then reassembling the full quark path on lookup.
The previous always-False stubs sent libXt's _XtDisplayInitialize into an alloca-doubling loop until the stack smashed.

mk/libxt.mk compiles util/makestrs.c as a host tool, runs it to generate StringDefs.c, StringDefs.h, and Shell.h, then compiles the 53 libXt translation units with -DHAVE_CONFIG_H -DLIBXT_COMPILATION -DXT_NO_SM. include/libxt-build/config.h is the synthesized autoconf output. include/X11/Xmu/Editres.h, include/X11/SM/SMlib.h, include/X11/ICE/ICElib.h satisfy unconditional libXt header includes without dragging libXmu, libSM, or libICE into the link closure.

scripts/sync-upstream-headers.py gains per-source src_take_all and util_files knobs, plus a SRC_PATCHES entry that works around a real upstream bug in libXt-1.3.1's SessionSetValues, where a misplaced XT_NO_SM #endif leaves widget casts referenced outside their declaration scope.

---

Summary by cubic

Builds libXt-compat.so from pinned libXt-1.3.1 and wires it into the build, enabling Xt apps to run on our SDL2-backed Xlib without new system deps. Implements missing Xlib hooks and a robust Xrm quark bridge so initialization and resource lookups work correctly, including deep prefixes.

• New Features

  • Build libXt-compat.so exporting 299 Xt symbols; links only to libX11-compat.so and libSystem.
  • Bridge Xrm quark APIs (XrmQGetResource, XrmQGetSearchList, XrmQGetSearchResource) with prefix encoding and correct buffer-size semantics to unblock _XtDisplayInitialize and support deep resource paths.
  • Add XConvertCase (from upstream libX11) for correct keysym case folding.
  • Stub internal-connection hooks (XAddConnectionWatch, XRemoveConnectionWatch, XInternalConnectionNumbers, XProcessInternalConnection) for the SDL backend.
  • Fix XrmParseCommand to only NULL-terminate argv when compression frees a slot, avoiding an overrun; matches upstream behavior.
  • Add libXt smoke and micro tests (test-libxt-link, test-libxt-micro) covering init, events, callbacks, GC, and fallback resources; integrated into make check.

• Dependencies

  • Add mk/libxt.mk; compile 53 upstream sources, build host makestrs to generate StringDefs.{c,h} and Shell.h, and link with -DXT_NO_SM.
  • Stage generated StringDefs.h/Shell.h under the staged X11 include tree so <X11/Shell.h> resolves for downstream consumers.
  • Add stub headers X11/Xmu/Editres.h, X11/SM/SMlib.h, X11/ICE/ICElib.h to avoid pulling libXmu, libSM, or libICE.
  • Add include/libxt-build/config.h (enables USE_POLL, XTHREADS; disables SM).
  • Extend scripts/sync-upstream-headers.py with per-source src_take_all/util_files, stage libXt src/util into separate dirs, and patch libXt-1.3.1 Shell.c SessionSetValues for XT_NO_SM.
  • Include mk/libxt.mk from the top-level Makefile; add test rules linking libXt-compat.so and libX11-compat.so. On Linux, add -Wl,-rpath,'$ORIGIN' for runtime and -Wl,-rpath-link,$OUT on tests.

^{Written for commit 8ba599c. Summary will update on new commits.}