Remove dead MSVC version fallbacks in CGAL_GeneratorSpecificSettings.cmake

Remove the dead elseif(MSVC14) through elseif(MSVC71) fallback chain.
These are unreachable because CGAL requires CMake >= 3.15, which always
defines MSVC_TOOLSET_VERSION for any MSVC build. Additionally, CGAL
requires C++17, which needs VS 2017+ at minimum.

Add an explicit elseif(MSVC) branch with AUTHOR_WARNING for the edge
case where MSVC is detected but MSVC_TOOLSET_VERSION is unexpectedly
empty, so it fails loudly instead of silently skipping toolset config.

The else() branch preserves the status message for non-MSVC compilers.

Author: RajdeepKushwaha5

Installation/cmake/modules/CGAL_GeneratorSpecificSettings.cmake

@@ -11,27 +11,11 @@ if ( NOT CGAL_GENERATOR_SPECIFIC_SETTINGS_FILE_INCLUDED )
   if ( MSVC_TOOLSET_VERSION )
     set(CGAL_TOOLSET "vc${MSVC_TOOLSET_VERSION}")
     message( STATUS "Using VC toolset ${MSVC_TOOLSET_VERSION}." )
-  elseif ( MSVC14 )
-    set(CGAL_TOOLSET "vc140")
-    message( STATUS "Using VC14 compiler." )
-  elseif ( MSVC12 )
-    set(CGAL_TOOLSET "vc120")
-    message( STATUS "Using VC12 compiler." )
-  elseif ( MSVC11 )
-    set(CGAL_TOOLSET "vc110")
-    message( STATUS "Using VC11 compiler." )
-  elseif ( MSVC10 )
-    set(CGAL_TOOLSET "vc100")
-    message( STATUS "Using VC10 compiler." )
-  elseif ( MSVC90 )
-    set(CGAL_TOOLSET "vc90")
-    message( STATUS "Using VC90 compiler." )
-  elseif ( MSVC80 )
-    set(CGAL_TOOLSET "vc80")
-    message( STATUS "Using VC80 compiler." )
-  elseif ( MSVC71 )
-    set(CGAL_TOOLSET "vc71")
-    message( STATUS "Using VC71 compiler." )
+  elseif ( MSVC )
+    message( AUTHOR_WARNING
+      "MSVC detected but MSVC_TOOLSET_VERSION is not set. "
+      "CGAL_TOOLSET will not be configured. "
+      "This may indicate an unsupported CMake version or toolchain." )
   else()
     message( STATUS "Using ${CMAKE_CXX_COMPILER} compiler." )
   endif()