remove C++20 constraint in CMakeLists

Author: gouarin

CMakeLists.txt

@@ -12,17 +12,6 @@ project(xtl)
 
 set(XTL_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
 
-# Enforce C++20
-if(DEFINED CMAKE_CXX_STANDARD)
-    if(CMAKE_CXX_STANDARD LESS 20)
-        message(FATAL_ERROR "xtl requires at least C++20")
-    endif()
-else()
-    set(CMAKE_CXX_STANDARD 20)
-endif()
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
-
 # Versioning
 # ===========