Update CMakeLists

- remove GOLD linker special case handling
- bump minimum cmake version to 3.20
- simplify LTO detection
- remove outdated way to check for compiler flags
- remove outdated compiler version gates (rely on CXX version)
- remove outdated Apple platform handling
- disable compiler-specific CXX extensions
- require boost 1.70 in all builds
- remove unused variable

cmake/CheckCXXCompilerFlag.cmake

@@ -1,29 +0,0 @@
-# - Check whether the CXX compiler supports a given flag.
-# CHECK_CXX_COMPILER_FLAG(<flag> <var>)
-#  <flag> - the compiler flag
-#  <var>  - variable to store the result
-# This internally calls the check_cxx_source_compiles macro.  See help
-# for CheckCXXSourceCompiles for a listing of variables that can
-# modify the build.
-
-# Copyright (c) 2006, Alexander Neundorf, <neundorf@kde.org>
-#
-# Redistribution and use is allowed according to the terms of the BSD license.
-# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
-
-
-INCLUDE(CheckCXXSourceCompiles)
-
-MACRO (CHECK_CXX_COMPILER_FLAG _FLAG _RESULT)
-   SET(SAFE_CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS}")
-   SET(CMAKE_REQUIRED_DEFINITIONS "${_FLAG}")
-   CHECK_CXX_SOURCE_COMPILES("int main() { return 0;}" ${_RESULT}
-     # Some compilers do not fail with a bad flag
-     FAIL_REGEX "unrecognized .*option"                     # GNU
-     FAIL_REGEX "ignoring unknown option"                   # MSVC
-     FAIL_REGEX "[Uu]nknown option"                         # HP
-     FAIL_REGEX "[Ww]arning: [Oo]ption"                     # SunPro
-     FAIL_REGEX "command option .* is not recognized"       # XL
-     )
-   SET (CMAKE_REQUIRED_DEFINITIONS "${SAFE_CMAKE_REQUIRED_DEFINITIONS}")
-ENDMACRO (CHECK_CXX_COMPILER_FLAG)

cmake/JSONParser.cmake

@@ -1,7 +1,7 @@
 # https://github.com/sbellus/json-cmake/blob/9913da8800b95322d393894d3525d634568f305e/JSONParser.cmake
 # MIT Licensed - https://github.com/sbellus/json-cmake/blob/master/LICENSE
 
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.20)
 
 if (DEFINED JSonParserGuard)
     return()