make libunwind portability header

Summary: Adds a portability header for later ease of use when deciding to include external_deps libunwind in TARGETS files

Reviewed By: yfeldblum

Differential Revision: D27416623

fbshipit-source-id: fc42f010302b8caa280fdf5cbdf013ed1b23af04

folly/experimental/symbolizer/StackTrace.cpp

@@ -26,7 +26,7 @@
 #if FOLLY_HAVE_LIBUNWIND
 // Must be first to ensure that UNW_LOCAL_ONLY is defined
 #define UNW_LOCAL_ONLY 1
-#include <libunwind.h>
+#include <folly/portability/Libunwind.h>
 #endif
 
 #if FOLLY_HAVE_BACKTRACE

folly/portability/Libunwind.h

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#if __has_include(<libunwind.h>)
+#include <libunwind.h> // @manual
+#endif