io.c: buffer must be modifiable

* io.c (io_setstrbuf): always check if the buffer is modifiable.
  [ruby-core:62643] [Bug #9847]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: add NULLCMD

* lib/mkmf.rb (configuration): add NULLCMD.

* lib/mkmf.rb (create_makefile): use NULLCMD macro.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45980 b2dd03c8-39d4-4d8f-98ff-823fe69b080e