* configure.in: check strcasecmp().

* include/ruby/ruby.h: use strcasecmp() as st_strcasecmp() if it
  exists.

* st.c (st_strcasecmp): define the function only if strcasecmp()
  doesn't exist.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42008 b2dd03c8-39d4-4d8f-98ff-823fe69b080e