configure.in: rb_cv_atan2_inf_c99

* configure.in (rb_cv_atan2_inf_c99): set `no` if atan2f and
  atan2l are not available, and quote the result.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45998 b2dd03c8-39d4-4d8f-98ff-823fe69b080e