libstdc++: Use more descriptive feature test macro

libstdc++: Use more descriptive feature test macro

The out-of-class definitions of the static constants are redundant if
the __cpp_inline_variables feature is supported, so use that macro to
decide whether to define them or not.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h: Check __cpp_inline_variables instead of
	__cplusplus.