* common.mk: ditto.
* configure.in: ditto.
* dln_find.c: ditto.
* ext/Setup.emx: ditto.
* ext/extmk.rb: ditto.
* ext/socket/extconf.rb: ditto.
* ext/zlib/extconf.rb: ditto.
* file.c: ditto.
* include/ruby/defines.h: ditto.
* io.c: ditto.
* lib/mkmf.rb: ditto.
* missing/os2.c: ditto.
* process.c: ditto.
* ruby.c: ditto.
* NEWS: announce OS/2 is no longer supported.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52161 b2dd03c8-39d4-4d8f-98ff-823fe69b080e