mkmf.rb: suppress warnings

* lib/mkmf.rb (try_func, try_var, scalar_ptr_type, scalar_type):
  suppress implicit-function-declaration warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40224 b2dd03c8-39d4-4d8f-98ff-823fe69b080e