actually check its availability rather to check GCC's version.

	* configure.in (WARN_UNUSED_RESULT): moved to here.

	* configure.in (RUBY_FUNC_ATTRIBUTE): change function declaration
	  to return int rather than void, because it makes no sense for a
	  warn_unused_result attributed function to return void.

	  Funny thing however is that it also makes no sense for noreturn
	  attributed function to return int.  So there is a fundamental
	  conflict between them.  While I tested this, I confirmed both
	  GCC 6 and Clang 3.8 prefers int over void to correctly detect
	  necessary attributes under this setup.  Maybe subject to change
	  in future.

	* internal.h (UNINITIALIZED_VAR): renamed to MAYBE_UNUSED, then
	  moved to configure.in for the same reason we move
	  WARN_UNUSED_RESULT.

	* configure.in (MAYBE_UNUSED): moved to here.

	* internal.h (__has_attribute): deleted, because it has no use now.

	* string.c (rb_str_enumerate_lines): refactor macro rename.

	* string.c (rb_str_enumerate_bytes): ditto.

	* string.c (rb_str_enumerate_chars): ditto.

	* string.c (rb_str_enumerate_codepoints): ditto.

	* thread.c (do_select): ditto.

	* vm_backtrace.c (rb_debug_inspector_open): ditto.

	* vsnprintf.c (BSD_vfprintf): ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56169 b2dd03c8-39d4-4d8f-98ff-823fe69b080e