Append '.gemspec' extension only when it is not present.

This avoids confusing messages such as:

~~~
$ gem build *.gemspec
ERROR:  Gemspec file not found: *.gemspec.gemspec
~~~

Fixes #3953

lib/rubygems/commands/build_command.rb

@@ -79,7 +79,7 @@ def find_gemspec
   end
 
   def build_gem(gem_name)
-    gemspec = File.exist?(gem_name) ? gem_name : "#{gem_name}.gemspec"
+    gemspec = File.extname(gem_name) == ".gemspec" ? gem_name : "#{gem_name}.gemspec"
 
     if File.exist?(gemspec)
       spec = Gem::Specification.load(gemspec)