[Patch, libstdc++/65420] Use constexpr variables as regex_constans flags

[Jonathan Wakely]

On 15/03/15 23:27 -0700, Tim Shen wrote:
>> All it needs is something like:
>>
>> enum syntax_option_type : unsigned { };
>> constexpr syntax_option_type icase = ...;
>
>Acknowledged.
>
>Here's the simple version of it.

OK for trunk, thanks.