cmake: remove dead compiler/platform flag blocks from CGAL_SetupCGALDependencies

[RajdeepKushwaha5]

Summary of Changes

Follow-up to #9356. As @lrineau noted there, more obsolete code remained in the compiler-flag setup. This PR removes four dead blocks from CGAL_SetupCGALDependencies.cmake:

1. SunPro compiler block — Oracle/Sun Studio has been EOL since 2012 and does not support C++17 (required by CGAL 6.0). The block set STLPort 4 flags that have no modern users.

2. GCC 4.2 -fno-strict-aliasing block — GCC 4.2 (2007) cannot compile C++17. Additionally, the GCC_VERSION variable it reads is only populated by the legacy FLAGS section that Remove old CMake code about gcc version check for -frounding-math (#6923 ) #9356 removes.

3. DEC Alpha processor flags — The Alpha architecture was discontinued circa 2004. The -mieee/-mfp-rounding-mode=d flags are irrelevant to any modern platform.

4. Intel CMAKE_VERSION VERSION_LESS 3.3 branch — CGAL requires CMake ≥ 3.12, so this condition is never true. Simplified to keep only the generator-expression code path.

The live MSVC, Intel (modern path), and GCC ≥ 4 flag blocks are preserved unchanged.

Release Management

• Affected package(s): Installation
• Issue(s) solved (if any): relates to Remove old CMake code about gcc version check for -frounding-math  #6923
• License and copyright ownership: same as existing file

[Copilot]

Pull request overview

This PR continues the cleanup from #9356 by removing additional obsolete/dead compiler/platform flag branches from CGAL_SetupCGALDependencies.cmake, simplifying CGAL’s CMake flag setup in line with the project’s C++17 and modern CMake requirements.

Changes:

• Simplified the Intel compiler flag logic by removing the unreachable CMAKE_VERSION VERSION_LESS 3.3 branch.
• Removed obsolete flag blocks for SunPro (STLPort), GCC 4.2-specific aliasing flags, and DEC Alpha processor flags.

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