configure.in, file.c: RUBY_FUNCTION_NAME_STRING

* configure.in (rb_cv_function_name_string): macro for function name
  string predefined identifier, __func__ in C99, or __FUNCTION__ in
  gcc.
* file.c (rb_sys_fail_path): use RUBY_FUNCTION_NAME_STRING.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e