Pin CMAKE_OSX_DEPLOYMENT_TARGET so the macOS floor is deliberate

[ZayanKhan-12]

Fixes #2565.

Nothing in the build set CMAKE_OSX_DEPLOYMENT_TARGET, so the macOS floor of every binary was an accident of the build machine's OS/SDK, and post-floor symbols linked cleanly but failed at load time on older Macs — #2564 (quick_exit) was one instance.

What this does

• Defaults the floor to 14.0 (the oldest macOS in the CI matrix), as a user-overridable cache variable set before the first PROJECT() call in both Superbuild/CMakeLists.txt and src/CMakeLists.txt. Guarded on CMAKE_HOST_APPLE (the target APPLE var doesn't exist before project()); ignored off-Apple.
• Forwards it explicitly to every compiled ExternalProject, since ExternalProject sub-builds do not inherit it:
  • CMake-configured externals (Zlib, SQLite, Teem, Freetype, Cleaver2, LodePng, Tny, Qwt, Tetgen, Ospray) get a cache/args entry mirroring the existing CMAKE_BUILD_TYPE forwarding.
  • Boost: the real compile happens in the build_libs b2 step, so b2 additionally gets cxxflags=/linkflags=-mmacosx-version-min=….
  • CPython: MACOSX_DEPLOYMENT_TARGET is set in the configure env; configure bakes it into its Makefile so the build steps inherit it.
  • Header-only externals (Eigen, GLM, SpdLog) and WIN32-only Glew unchanged. GoogleTestExternal.cmake is not referenced by the Superbuild, so it was left alone.
• Adds -Wunguarded-availability-new to the APPLE warning set in SCIRUN_ADD_LIBRARY, so availability-annotated post-floor API use is a compile-time warning instead of a runtime dyld failure. (Honest caveat: it would not have caught quick_exit, which the 15 SDK declares without availability annotations — but with the pin, __MAC_OS_X_VERSION_MIN_REQUIRED now reads 140000 deterministically, which is what makes Fix dyld crash on macOS < 15 from unguarded quick_exit reference #2564's guard reliable.)
• Adds a macOS CI step asserting the built binary's LC_BUILD_VERSION minos equals the pin, so a dropped flag shows up as a red build rather than a user crash report.

Verification

• Superbuild configure run locally (Apple Silicon, CMake 4.3): CMAKE_OSX_DEPLOYMENT_TARGET:STRING=14.0 lands in the top-level cache and in every generated Externals/tmp/<name>_external-cache-Release.cmake plus Tetgen's configure command.
• The new CI otool step verifies the end-to-end result on the macOS PR job.

Note: touches the same ExternalProject_Add blocks as the superbuild-caching residue on #2513 — happy to rebase whichever lands second.

[dcwhite]

I'll have to examine this one carefully (not just with Claude 😉 )