fix bugs on building extensions

mikunyan.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = 'exe'
   spec.executables   = spec.files.grep(%r{^exe/}){|f| File.basename(f)}
   spec.require_paths = ['lib']
-  spec.extensions    = ['ext/decoders/native/extconf.rb']
+  spec.extensions    = ['ext/decoders/native/extconf.rb', 'ext/decoders/crunch/extconf.rb']
 
   spec.add_dependency 'bin_utils', '~> 0'
   spec.add_dependency 'chunky_png', '~> 1'