* internal.h: use __builtin_bswap16() if possible.

* configure.in: check existence of __builtin_bswap16().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43831 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-11-25

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43832 b2dd03c8-39d4-4d8f-98ff-823fe69b080e