build: include libmtmd in Apple XCFramework (#21935)

Adds an opt-in LLAMA_BUILD_MTMD CMake option so build-xcframework.sh
can link libmtmd.a into the framework binary without pulling in the
rest of tools/ (which doesn't cross-build cleanly to iOS/tvOS/visionOS).

- CMakeLists.txt: new option, default OFF. When on with
  LLAMA_BUILD_TOOLS=OFF, only the tools/mtmd subdir is added. Useful
  for any binding that wants just libmtmd (Apple XCFramework, WASM).
- tools/mtmd/CMakeLists.txt: gate the CLI exe targets on
  LLAMA_BUILD_TOOLS. Gating on LLAMA_BUILD_COMMON is not enough — it
  defaults ON in standalone builds and visionOS xcodebuild then fails
  with "install TARGETS given no BUNDLE DESTINATION for MACOSX_BUNDLE
  executable target 'llama-mtmd-cli'".
- build-xcframework.sh: turn the option on, pass -DLLAMA_BUILD_MTMD,
  add libmtmd.a to combine_static_libraries, and copy mtmd.h and
  mtmd-helper.h into the framework Headers dir. The umbrella module
  map then exposes them, so Swift / Obj-C consumers can import the
  mtmd C API directly.

After this, nm on ios-arm64/llama.framework/llama shows 52 _mtmd_
symbols. Verified end-to-end: a Swift target links the produced
framework and calls mtmd_default_marker, mtmd_bitmap_init, etc.
without a shim on macos / iphoneos / iphonesimulator / xros slices.

Co-authored-by: Abraham Gonzalez <abraham@theabecaster.com>

CMakeLists.txt

@@ -222,6 +222,16 @@ if (LLAMA_BUILD_APP)
     add_subdirectory(app)
 endif()
 
+# Standalone libmtmd build without pulling in the rest of the tools/ tree.
+# Useful when packaging just the mtmd library for language bindings (e.g. an
+# Apple XCFramework, or a WASM build). When the full tools build is enabled,
+# mtmd is already built by the tools/ subdirectory above; this hook only fires
+# when LLAMA_BUILD_TOOLS is OFF to avoid double-adding the target.
+option(LLAMA_BUILD_MTMD "llama: build tools/mtmd library standalone" OFF)
+if (LLAMA_BUILD_MTMD AND NOT (LLAMA_BUILD_COMMON AND LLAMA_BUILD_TOOLS))
+    add_subdirectory(tools/mtmd)
+endif()
+
 #
 # install
 #