cmake: remove all dead compiler-specific flag blocks from CMakeLists.txt

All compiler flag logic for MSVC, SunPro, Intel Classic (icl/icpc), and
GCC was already migrated to the modern target-based mechanism in
Installation/cmake/modules/CGAL_SetupCGALDependencies.cmake, which uses
target_compile_options()/target_link_libraries() INTERFACE. The old
blocks used uniquely_add_flags(CGAL_CXX_FLAGS ...) which is the
obsolete global-variable approach.

Removed dead blocks:
- MSVC: /fp:strict, /fp:except-, /bigobj, _SCL_SECURE_NO_DEPRECATE,
  CMAKE_CXX_WARNING_LEVEL (all superseded by target_compile_options in
  CGAL_setup_CGAL_flags())
- SunPro: -features=extensions, -library=stlport4, -D_GNU_SOURCE,
  linker flags (superseded by target_compile_options +
  target_link_libraries in CGAL_setup_CGAL_flags())
- Intel Classic (icl/icpc): /fp:strict / -fp-model=strict, version
  check for < 11.0 (superseded; Intel < 11.0 from 2008 cannot support
  the C++17 that CGAL now requires)
- GCC: -Wall for test suite, -frounding-math regex check, -g stripping
  from CMAKE_CXX_FLAGS, -mieee -mfp-rounding-mode=d for alpha
  (all superseded by target_compile_options in CGAL_setup_CGAL_flags())

The entire empty FLAGS section header is also removed.

Closes #6923

Author: RajdeepKushwaha5

Installation/CMakeLists.txt

@@ -387,97 +387,6 @@ message(
 )
 message(STATUS "System: ${CMAKE_SYSTEM_NAME}")
 
-#--------------------------------------------------------------------------------------------------
-#
-#                                    -= FLAGS =-
-#
-#--------------------------------------------------------------------------------------------------
-
-if(MSVC)
-
-  uniquely_add_flags(CGAL_CXX_FLAGS
-                     "-D_SCL_SECURE_NO_DEPRECATE;-D_SCL_SECURE_NO_WARNINGS")
-  uniquely_add_flags(CGAL_CXX_FLAGS "/fp:strict")
-  uniquely_add_flags(CGAL_CXX_FLAGS "/fp:except-")
-  uniquely_add_flags(CGAL_CXX_FLAGS "/bigobj") # Use /bigobj by default
-
-  if(RUNNING_CGAL_AUTO_TEST OR CGAL_TEST_SUITE)
-    set(CMAKE_CXX_WARNING_LEVEL
-        2
-        CACHE STRING "MSVC C++ compiler warning level" FORCE)
-    mark_as_advanced(CMAKE_CXX_WARNING_LEVEL)
-  endif()
-
-endif()
-
-if("\"${CMAKE_CXX_COMPILER_ID}\"" MATCHES "SunPro")
-  message(STATUS "Using SunPro compiler, using STLPort 4.")
-
-  uniquely_add_flags(CGAL_CXX_FLAGS
-                     "-features=extensions;-library=stlport4;-D_GNU_SOURCE")
-  uniquely_add_flags(CGAL_SHARED_LINKER_FLAGS "-library=stlport4")
-  uniquely_add_flags(CGAL_EXE_LINKER_FLAGS "-library=stlport4")
-endif()
-
-if("${CMAKE_CXX_COMPILER}" MATCHES "icl" OR "${CMAKE_CXX_COMPILER}" MATCHES
-                                            "icpc")
-  message(STATUS "Intel compiler is detected.")
-  set(IntelCompiler_FOUND TRUE)
-
-  get_dependency_version(IntelCompiler)
-
-  if("${IntelCompiler_VERSION}" LESS "1100")
-    message(
-      "Intel Compiler version ${IntelCompiler_VERSION} is not supported by CGAL-${CGAL_MAJOR_VERSION}.${CGAL_MINOR_VERSION}! (too old, must be 11.0 or after)"
-    )
-  else()
-    message(
-      STATUS "Using Intel Compiler version 11 (or greater). Adding -fp-model=strict"
-    )
-    if(WIN32)
-      uniquely_add_flags(CGAL_CXX_FLAGS "/fp:strict")
-    else()
-      uniquely_add_flags(CGAL_CXX_FLAGS "-fp-model=strict")
-    endif()
-  endif()
-endif()
-
-if(CMAKE_COMPILER_IS_GNUCXX)
-
-  set(GCC_FOUND TRUE)
-
-  get_dependency_version(GCC)
-
-  if("${GCC_VERSION}" MATCHES "Not" OR "${GCC_VERSION}" MATCHES "Unknown")
-    set(GCC_FOUND FALSE)
-  endif()
-
-  if(GCC_FOUND)
-
-    if(RUNNING_CGAL_AUTO_TEST OR CGAL_TEST_SUITE)
-      uniquely_add_flags(CGAL_CXX_FLAGS "-Wall")
-      # Remove -g from the relevant CMAKE_CXX_FLAGS. This will also
-      # propagate to the rest of the tests, since we overwrite those
-      # flags with the ones used to build CGAL.
-      string(REGEX REPLACE "-g( |$)" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-      # We only allow the release types DEBUG and RELEASE, but handle
-      # all possible values just to be sure.
-      foreach(release_type DEBUG RELEASE MINSIZEREL RELWITHDEBINFO)
-        string(REGEX REPLACE "-g( |$)" "" CMAKE_CXX_FLAGS_${release_type}
-                             "${CMAKE_CXX_FLAGS_${release_type}}")
-      endforeach()
-
-    endif()
-
-    if("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "alpha")
-      message(STATUS "Using gcc on alpha. Adding -mieee -mfp-rounding-mode=d")
-      uniquely_add_flags(CGAL_CXX_FLAGS "-mieee -mfp-rounding-mode=d")
-    endif()
-
-  endif()
-
-endif()
-
 if(CGAL_TEST_SUITE)
   include(display-third-party-libs-versions)
 endif()