mswin: static-linked-ext

* enc/depend (CPPFLAGS), lib/mkmf.rb (MakeMakefile#create_makefile):
  define RUBY_EXPORT for static-linked-ext mswin.  [Bug #7960]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39556 b2dd03c8-39d4-4d8f-98ff-823fe69b080e