Add spec.extensions

Author: mrkn

pycall.gemspec

@@ -27,6 +27,7 @@ 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/pycall/extconf.rb"]
 
   spec.add_development_dependency "bundler", "~> 1.13"
   spec.add_development_dependency "rake", "~> 10.0"