configure.in: do not disable ucontext.h

* configure.in (ac_cv_func_{getcontext,setcontext}): do not
  disable ucontext.h entirely, but disable use of functions only.
  `ucontext_t` is necessary in the signal handler now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46114 b2dd03c8-39d4-4d8f-98ff-823fe69b080e