Use ENV['RUBY_EXE'] as RbConfig.ruby might be incorrect

* On a built-but-not-installed-ruby.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60529 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/optional/capi/spec_helper.rb

@@ -27,7 +27,7 @@ def compile_extension(name)
   ruby_library = "#{RbConfig::CONFIG['libdir']}/#{libruby_so}"
   unless libruby_so and File.exist?(ruby_library)
     # Statically-compiled lib in the binary
-    ruby_library = RbConfig.ruby
+    ruby_library = ENV['RUBY_EXE']
   end
 
   return lib if File.exist?(lib) and