build folly/experimental/symbolizer:stack_trace on mac

Summary:
ssize_t is not standard, so include <sys/types.h>. (I considered
switching to intptr_t but decided against it.)

Also, on macOS, the function appears to be called backtrace, not
unw_backtrace.

Reviewed By: yfeldblum

Differential Revision: D14307252

fbshipit-source-id: d919bd37fea8f68b5d610e771e577c8ce385f6ec

folly/experimental/symbolizer/StackTrace.cpp

@@ -19,6 +19,10 @@
 #define UNW_LOCAL_ONLY 1
 #include <libunwind.h>
 
+#ifdef __APPLE__
+#include <execinfo.h>
+#endif
+
 namespace folly {
 namespace symbolizer {
 
@@ -28,7 +32,11 @@ ssize_t getStackTrace(uintptr_t* addresses, size_t maxAddresses) {
   // The libunwind documentation says that unw_backtrace is async-signal-safe
   // but, as of libunwind 1.0.1, it isn't (tdep_trace allocates memory on
   // x86_64)
+#ifdef __APPLE__
+  int r = backtrace(reinterpret_cast<void**>(addresses), maxAddresses);
+#else
   int r = unw_backtrace(reinterpret_cast<void**>(addresses), maxAddresses);
+#endif
   return r < 0 ? -1 : r;
 }
 

folly/experimental/symbolizer/StackTrace.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <sys/types.h>
 #include <cstdint>
 #include <cstdlib>