Fix superbuild flag propagation and support StealthLink on Linux

[mkomaiha]

@lassoan @Sunderlandkyl
Fixes #957 (together with PlusToolkit/PlusLib#1292).

Four superbuild fixes found while building with PLUS_USE_STEALTHLINK=ON on Linux. The first three are general and independent of StealthLink — each can be cherry-picked on its own. The fourth builds on the third.

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1. COMP: Enforce CMake 3.5 min for VTK

CMake 4 removed compatibility with cmake_minimum_required below 3.5. VTK 9.1 vendors a KWSys whose minimum predates that, so VTK cannot configure at all under CMake 4:

CMake Error at Utilities/KWSys/vtksys/CMakeLists.txt:91 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

Adds -DCMAKE_POLICY_VERSION_MINIMUM=3.5, the same treatment already applied to tesseract (26567be) and aruco (509f6bb).

2. BUG: Forward CMAKE_<LANG>_FLAGS to the external projects

ep_common_c_flags / ep_common_cxx_flags were built from CMAKE_<LANG>_FLAGS_INIT, which holds only the toolchain defaults. Flags passed on the command line land in CMAKE_<LANG>_FLAGS and were silently dropped, so every external project was built in a different configuration than the superbuild was asked for — with no diagnostic.

External_ITK.cmake had the same defect from the other side: itk_common_cxx_flags was assigned from ep_common_cxx_flags only under UNIX AND NOT APPLE, so macOS passed ITK no CXX flags at all, and the MSVC branch appended /MP to a variable that had never been set — dropping them there too.

This is the root cause of the ABI errors in #957: -D_GLIBCXX_USE_CXX11_ABI=0 never reached VTK, ITK, IGSIO or OpenIGTLink, so they were built with a different std::string than the SDK.

3. BUG: Resolve the ITK version before ep_common_args is built

ep_common_args captures CMAKE_CXX_STANDARD, but the block raising it to 17 for ITK 5 ran after, so external projects received the pre-bump value. On a clean single-pass configure ITK 5.4 is handed CMAKE_CXX_STANDARD=14 and rejects it:

CMake Error at CMakeLists.txt:39 (message):
  C++98 to C++14 are no longer supported in ITK version 5.4 and greater

Reconfiguring hides this, because the second pass reads the bumped value back from the cache — which is why it survives in-tree. The resolution depends only on PLUS_USE_STEALTHLINK and MSVC, both known well before ep_common_args, so it simply moves up.

4. ENH: Support building StealthLink on Linux

Depends on #3 (it edits the block that commit moves).

• Set the ABI flag centrally. The Linux SDK is built against the pre-GCC5 libstdc++ ABI, and everything exchanging std::string with it must agree — not just PlusLib but VTK, ITK, IGSIO and OpenIGTLink. This was previously left to the user via a warning. -D_GLIBCXX_USE_CXX11_ABI=0 is now set once, before ep_common_cxx_flags is built; the warning is removed.
• Pin ITK 4 only under MSVC. That requirement comes from StealthLink being limited to the Visual Studio 2013 runtime. On Linux the constraint is the libstdc++ ABI, now handled above, and forcing ITK 4 is actively harmful: ITK 4.12 vendors a 2017-era VXL whose compiler check stops at GCC 7 and fails with #error "Dunno about this gcc" on anything newer.
• Forward STEALTHLINK_STEALTHLINK_SHARED_LIBRARY on every platform, since PlusLib links it on Linux (Fix two Linux build failures: shared StealthLink library and unused vtkImageViewer2 include PlusLib#1292).
• Fail clearly on macOS. The SDK ships only Windows and Linux binaries; FindSTEALTHLINK previously fell through both platform branches and reported the SDK as missing.

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Testing

Ubuntu 21.04, GCC 10, CMake 4.4.2, VTK 9.1.0, ITK 5.4.4, StealthLink SDK 2.4.

Each failure above reproduces without its commit. With all four applied, a single-pass configure gives every external project a consistent standard and ABI:

vtk:           CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0
itk:           CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0
IGSIO:         CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0
OpenIGTLink:   CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0
OpenIGTLinkIO: CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0
PlusLib:       CMAKE_CXX_STANDARD=17  -D_GLIBCXX_USE_CXX11_ABI=0

and the built libraries carry the old ABI, e.g. vtksys::SystemTools::FileIsFullPath mangles as ...ERKSs (const std::string&) rather than ...ERKNSt7__cxx11..., with zero __cxx11 symbols in libvtksys.

The build then completes and the device is available at runtime:

$ PlusVersion | grep -i stealth
  - StealthLinkTracker (ver: Plus-2.9.0)

Note that reaching a successful link also requires PlusToolkit/PlusLib#1292 (PlusLib links the static SDK library, which carries none of the SDK's boost dependencies) and IGSIO/IGSIO#62 (vtkAddon was built with the opposite ABI), both of which are separate from this change. IGSIO#62 and IGSIO#63 are merged.