Fix the logic of selection of std::regex

Fix the logic of selection of std::regex

This amends commit e1938aa2, which introduced some logic
errors: regex would be enabled for gcc versions which have unusable
<regex> header.

The new code separates better special gcc handling from special C++11
handling and should be more readable.

* set -std=c++11 in AM_CPPFLAGS instead of AM_CXXFLAGS, since the
  preprocessor uses this setting too.

* Before checking for header <regex>, set language to C++ and update
  value of AM_CPPFLAGS too.

* Separate code that checks for regex in its own macro.

* now unknown compiler which have the <regex> header, will use std::regex in C++11 mode.