defines.h: RUBY_SYMBOL_EXPORT_{BEGIN,END}

* include/ruby/defines.h (RUBY_SYMBOL_EXPORT_{BEGIN,END}): visibility
  control macros.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40122 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure.in: no -include options

* configure.in (XCFLAGS): stop including missing.h by command line
  option, as the header seems included necessary places already.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40123 b2dd03c8-39d4-4d8f-98ff-823fe69b080e