* configure.in: do not use buggy tgamma() of mingw.

* missing/tgamma.c (tgamma): merge fix for inifity from
  ruby_tgamma.  since msvcr120.dll and later have tgamma, this
  implementation will not be used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58691 b2dd03c8-39d4-4d8f-98ff-823fe69b080e