mkconfig.rb: embed version substring

* tool/mkconfig.rb (RbConfig): embed substring of the version, not
  at the runtime, and fall back to the version itself for the case
  it does not include the teeny version.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50094 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/mkconfig.rb

@@ -171,7 +171,7 @@ puts %[\
 # changes made to this file will be lost the next time ruby is built.
 
 module RbConfig
-  RUBY_VERSION.start_with?("#{version}"[/^[0-9]+[.][0-9]+[.]/]) or
+  RUBY_VERSION.start_with?("#{version[/^[0-9]+\.[0-9]+\./] || version}") or
     raise "ruby lib version (#{version}) doesn't match executable version (\#{RUBY_VERSION})"
 
 ]