Allow building with C++20 (#60)

Co-authored-by: jmcarcell <[email protected]>

CMakeLists.txt

@@ -33,7 +33,7 @@ endif()
 # ``-DCMAKE_CXX_STANDARD=<standard>`` when invoking CMake
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "")
 
-if(NOT CMAKE_CXX_STANDARD MATCHES "14|17")
+if(NOT CMAKE_CXX_STANDARD MATCHES "17|20")
   message(FATAL_ERROR "Unsupported C++ standard: ${CMAKE_CXX_STANDARD}")
 endif()