Sync gnulib regex with glibc

[Paul Eggert]

Adhemerval Zanella wrote:
> It does look like a correct change, but what I am worried from glibc
> standpoint is if it would require a compatibility implementation
> (potentially mapping RE_SYNTAX_{E}GREP to old definition on compat symbol).

I doubt whether that would be needed (or, as Zack writes, helpful) in this case. 
(Neither RE_SYNTAX_GREP nor RE_SYNTAX_EGREP are documented in any way, amusingly 
enough, so one cannot say we're changing documented behavior....)