[build] clean out model specs before building them

Summary:
Sometimes building the models segfaults mysteriously. I've observed in the
latest instance that `make -C infer/models clean` solved the issue so this is
likely a good idea.

Reviewed By: mbouaziz

Differential Revision: D4745924

fbshipit-source-id: cdb50d8

Makefile

@@ -120,6 +120,9 @@ infer: src_build
 ifeq ($(BUILD_JAVA_ANALYZERS),yes)
 	@$(MAKE) -C $(ANNOTATIONS_DIR)
 endif
+#	Delete existing specs so that they are not used during the analysis of models. Infer may
+#	segfault in some cases otherwise.
+	@$(MAKE) -C $(MODELS_DIR) clean_specs
 	@$(MAKE) -C $(MODELS_DIR) all
 
 .PHONY: clang_setup