configure.in: Werror-implicit-function-declaration

* configure.in (warnflags): -Werror-implicit-function-declaration
  haven't been used as-is, but always replaced with -Werror= or -W.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39214 b2dd03c8-39d4-4d8f-98ff-823fe69b080e