Fix some compilation problems in C++11 mode

Fix some compilation problems in C++11 mode

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.