The regex_constants::multiline constant is defined for non-strict C++11
and C++14 modes, on the basis that the feature is a DR (even though it
was really a new feature addition to C++17 and probably shouldn't have
gone through the issues list).

This makes the basic_regex::multiline constant defined consistently with
the regex_constants::multiline one.

For strict C++11 and C++14 mode we don't define them, because multiline
is not a reserved name in those standards.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (basic_regex::multiline): Define for
	non-strict C++11 and C++14 modes.
	* include/bits/regex_constants.h (regex_constants::multiline):
	Add _GLIBCXX_RESOLVE_LIB_DEFECTS comment.

Tested x86_64-linux. Committed to trunk.