Merge pull request #3984 from RosettaCommons/JackMaguire/CxxExtras
Add extras=cxx14, cxx17, and cxx20
Adding extras for newer c++ versions. This probably won't impact the end users much but it would help developers prototype with newer features or play with external libraries that require newer versions.
I also added macros like CXX14_OR_LATER because it seems cleaner than comparing __cplusplus to 201402L