Change std::regex detection test to detect bug in libstdc++.

libstdc++'s std::regex has (or had) a bug in std::regex::operator=(...) that
caused undefined behaviour. Clang will detect this and compile the function so
that it crashes at runtime. This patch tried to detect that bug during
configuration.

cmake/std_regex.cpp

@@ -2,9 +2,9 @@
 #include <string>
 int main() {
   const std::string str = "test0159";
-  const std::regex re(
-      "^[a-z]+[0-9]+$",
-      std::regex_constants::extended | std::regex_constants::nosubs);
+  std::regex re;
+  re = std::regex("^[a-z]+[0-9]+$",
+       std::regex_constants::extended | std::regex_constants::nosubs);
   return std::regex_search(str, re) ? 0 : -1;
 }