Use config file template instead of Rakefile template
diff --git a/build_config.rb b/build_config.rb
new file mode 100644
index 0000000..83edb1c
--- /dev/null
+++ b/build_config.rb
@@ -0,0 +1,3 @@
+module GrpcBuildConfig
+  CORE_WINDOWS_DLL = '/tmp/libs/opt/grpc-2.dll'
+end