C++11: Remove GnuRegex and all -lregex related code
Squid is now exclusively using the STL std::regex API provided on all operating systems in a portable manner.
We no longer have any need of detecting if the system is providing a libregex, or user has configured one, or if it actually works, or how to call it, or use the GnuRegex code as a backup when one of those complex details goes wrong.