Remove dead MSVC version fallbacks in CGAL_GeneratorSpecificSettings.cmake

[RajdeepKushwaha5]

Summary of Changes

Remove the dead elseif(MSVC14) through elseif(MSVC71) fallback chain in Installation/cmake/modules/CGAL_GeneratorSpecificSettings.cmake.

These fallbacks targeted Visual Studio 2003 (VC 7.1) through Visual Studio 2015 (VC14). They are unreachable dead code because:

1. CGAL requires CMake >= 3.15, and MSVC_TOOLSET_VERSION has been defined for all MSVC versions since CMake 3.12. The if(MSVC_TOOLSET_VERSION) branch always fires for any MSVC build, making the elseif chain unreachable.

2. CGAL requires C++17, which means Visual Studio 2017 (15.3) at minimum. None of the removed fallback versions (VS 2003–2015) can compile CGAL.

The else() branch (non-MSVC compilers) is preserved as elseif(NOT MSVC) to maintain the status message for GCC/Clang/etc.

Before:

if ( MSVC_TOOLSET_VERSION )
  set(CGAL_TOOLSET "vc${MSVC_TOOLSET_VERSION}")
  message( STATUS "Using VC toolset ${MSVC_TOOLSET_VERSION}." )
elseif ( MSVC14 )
  ...  # 7 dead elseif branches (VC14, VC12, VC11, VC10, VC90, VC80, VC71)
else()
  message( STATUS "Using ${CMAKE_CXX_COMPILER} compiler." )
endif()

After:

if ( MSVC_TOOLSET_VERSION )
  set(CGAL_TOOLSET "vc${MSVC_TOOLSET_VERSION}")
  message( STATUS "Using VC toolset ${MSVC_TOOLSET_VERSION}." )
elseif ( NOT MSVC )
  message( STATUS "Using ${CMAKE_CXX_COMPILER} compiler." )
endif()

Release Management

• Affected package(s): Installation
• Issue(s) solved (if any): contributes to Remove old CMake code about gcc version check for -frounding-math  #6923, Finish the modernization of CGAL CMake scripts #4815
• Feature/Small Feature (if any): N/A
• License and copyright ownership: N/A

[Copilot]

Pull request overview

This PR simplifies CGAL’s generator/toolset detection logic by removing legacy MSVC version fallback branches that are no longer relevant given CGAL’s current CMake/C++ requirements.

Changes:

• Removed the elseif(MSVC14) … elseif(MSVC71) fallback chain from CGAL_GeneratorSpecificSettings.cmake.
• Kept non-MSVC status reporting by changing the final branch to elseif(NOT MSVC).

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[Copilot]

With this change, if MSVC is true but MSVC_TOOLSET_VERSION is not set/empty, the script now produces no status output and leaves CGAL_TOOLSET unset (previously it would at least hit the else() message). If the intent is to rely on cmake_minimum_required(VERSION 3.15...) guaranteeing MSVC_TOOLSET_VERSION, consider adding an explicit elseif(MSVC) branch that errors or emits a clear diagnostic so unexpected toolchains/CMake versions fail loudly instead of silently skipping compiler/toolset reporting.

Suggested change

	elseif ( NOT MSVC )
	elseif ( MSVC )
	message(AUTHOR_WARNING
	"MSVC is detected but MSVC_TOOLSET_VERSION is not set. "
	"This may indicate an unsupported CMake version or toolchain; "
	"compiler/toolset reporting and CGAL_TOOLSET configuration may be incomplete.")
	else()