mkmf.rb: ignore linker warnings

	* lib/mkmf.rb (try_ldflags): ignore linker warnings.  they cause
	  unexpected failures on OpenBSD.  [ruby-core:78827] [Bug #13069]

	mkmf.rb: werror on mswin

	* lib/mkmf.rb (MakeMakefile#try_ldflags): enable warning checking
	  on mswin, link.exe warns -l options but does not fail.
	  [Bug #13069]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@65117 b2dd03c8-39d4-4d8f-98ff-823fe69b080e