RFC: make std::regex unusable

[Jonathan Wakely]

Can we make the constructor of std::basic_regex throw an exception, to
stop people thinking it's supposed to work and then being surprised or
annoyed that it's not implemented?

For people who want to experiment with it anyway or help implement it
we could suppress the exception if
_GLIBCXX_YES_I_KNOW_REGEX_DOES_NOT_WORK is defined.


Alternatively we could just stick a #warning in <regex>