do not clean inside facebook-clang-plugins/ if we are inside a release

Summary:Since we never build inside facebook-clang-plugins/ when we are inside a
release tree of infer, cleaning there would bork the tree. Then one would have
to unpack the release tarball again to unbork it. Moreover, it's very unclear
what's happening when you hit this issue. See #346

Reviewed By: jberdine

Differential Revision: D3207508

fb-gh-sync-id: 065a7f0
fbshipit-source-id: 065a7f0

Makefile.in

@@ -251,9 +251,11 @@ endif
 
 clean:
 	$(REMOVE) test.xml
+ifeq (@IS_RELEASE_TREE@,no)
 ifeq (@BUILD_C_ANALYZERS@,yes)
 	$(MAKE) -C $(FCP_DIR) clean
 	$(MAKE) -C $(FCP_DIR)/clang-ocaml clean
+endif
 endif
 	$(MAKE) -C $(INFER_DIR) clean