Add tiffdiff to CMake build system

[Copilot]

tiffdiff was built via the Visual Studio project files but omitted entirely from the CMake build, leaving it unbuildable on non-MSVC toolchains.

Changes

• cmake/Lcms2Options.cmake — Added LCMS2_BUILD_TIFFDIFF option (ON by default), consistent with the existing LCMS2_BUILD_TIFICC / LCMS2_BUILD_JPGICC pattern.

• cmake/Lcms2Tools.cmake — Added an optional tiffdiff build block after the tificc block. Shares the same TIFF dependency detection (LCMS2_WITH_TIFF / find_package(TIFF)) and links against TIFF::TIFF (or legacy TIFF_LIBRARIES). The target is registered in LCMS2_TOOL_TARGETS, so it is picked up by the packaging module for installation automatically.

# Optional tiffdiff (requires TIFF).
if(LCMS2_BUILD_TIFFDIFF)
  if(LCMS2_WITH_TIFF)
    find_package(TIFF)
  endif()
  if(TIFF_FOUND)
    _lcms2_add_tool(tiffdiff
      SOURCES
        "${PROJECT_SOURCE_DIR}/utils/tificc/tifdiff.c"
        ${_common_sources}
    )
    ...
  else()
    message(STATUS "TIFF not found or disabled; skipping tiffdiff")
  endif()
endif()

No changes outside the two CMake files.

[mm2]

everything seems ok