Improve formatting of the HAVE_WORKING_STD_REGEX test

Indent the test code in a nicer way and shorten it, taking advantage of
this guarantee from the C++ standard:

  If control reaches the end of main without encountering a return
  statement, the effect is that of executing return 0.

-> no need for "#include <cstdlib>" nor for "return EXIT_SUCCESS".

CMakeLists.txt

@@ -372,7 +372,6 @@ endif()
 #   [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78905
 check_cxx_source_compiles(
     "#include <regex>
-     #include <cstdlib>
 
      int main() {
        #if __cplusplus >= 201103L &&                             \
@@ -381,7 +380,6 @@ int main() {
             defined(_GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT) ||     \
             defined(_GLIBCXX_REGEX_STATE_LIMIT)           ||     \
             (defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE > 4))
-       return EXIT_SUCCESS;
        #else
          nullptr = void; // intentionally trigger a compilation error
        #endif