conditionally include __declspec to function definition to fix Window… (

facebookresearch#1584)

Summary:
…s DLL exports.

# Before submitting

- [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [x] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [x] Did you make sure to update the docs?
- [x] Did you write any new necessary tests?

## What does this PR do?
Fixes facebookresearch#292

☝️ technically this issue should remain open

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
👍
Pull Request resolved: facebookresearch#1584

Differential Revision: D19289792

Pulled By: myleott

fbshipit-source-id: 96af49e2ed808dde3f682906bc2d6e074b522c39

fairseq/clib/libbleu/libbleu.cpp

@@ -98,10 +98,16 @@ void bleu_addngram(
 
 extern "C" {
 
+#ifdef _WIN64
+__declspec(dllexport) 
+#endif
 void bleu_zero_init(bleu_stat* stat) {
   std::memset(stat, 0, sizeof(bleu_stat));
 }
 
+#ifdef _WIN64
+__declspec(dllexport) 
+#endif
 void bleu_one_init(bleu_stat* stat) {
   bleu_zero_init(stat);
   stat->count1 = 0;
@@ -114,6 +120,9 @@ void bleu_one_init(bleu_stat* stat) {
   stat->match4 = 1;
 }
 
+#ifdef _WIN64
+__declspec(dllexport) 
+#endif
 void bleu_add(
     bleu_stat* stat,
     size_t reflen, int* ref, size_t predlen, int* pred, int pad, int eos) {