Only enable std::regex when the header <regex> is present (fix Mac OS X issue).
Remove support for concept checks. The page
https://gcc.gnu.org/onlinedocs/libstdc++/manual/concept_checking.html
says about concept checking:
Please note that the checks are based on the requirements in the
original C++ standard, many of which were relaxed in the C++11
standard and so valid C++11 code may be incorrectly rejected by the
concept checks. Additionally, some correct C++03 code might be
rejected by the concept checks, for example template argument types
may need to be complete when used in a template definition, rather
than at the point of instantiation. There are no plans to address
these shortcomings.
Therefore it seems reasonable to remove our support for this.