link with -lpthread on linux

Rakefile.rb

@@ -27,6 +27,9 @@ task :test => :library do
                   "gtest-#{GTEST_VERSION}/src/gtest_main.cc",
                   "test/src/**/*.cc"]
     env.Program("build/tests", sources)
+    if RUBY_PLATFORM =~ /linux/
+      env["LIBS"] << "pthread"
+    end
   end
   system("./build/tests")
 end