* lib/mkmf.rb (try_cppflags, try_cflags, try_ldflags): get rid of
	  interference by modifying global variables in have_devel? method.
	  [ruby-core:67962] [Bug #10821]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@49993 b2dd03c8-39d4-4d8f-98ff-823fe69b080e