Upstream: https://github.com/bombela/backward-cpp/pull/221 (merged after v1.6)

Without it <dlfcn.h> is only included for the BFD and DW backends, while
Dl_info is used on every Linux build; glibc pulls the header in through
other system headers, musl does not.

From 65fc89e210655aaa707337bc21d19f45805ac8a2 Mon Sep 17 00:00:00 2001
From: Michael Truog <mjtruog@protonmail.com>
Date: Mon, 24 May 2021 23:09:51 -0700
Subject: [PATCH] Fix missing Dl_info type

* Dl_info is required when BACKWARD_SYSTEM_LINUX is defined
  (was a problem for BACKWARD_HAS_DW == 1)
---
 backward.hpp | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/backward.hpp b/backward.hpp
index 9ca22c5..b0b75e6 100644
--- a/backward.hpp
+++ b/backward.hpp
@@ -213,6 +213,13 @@
 #include <sys/stat.h>
 #include <syscall.h>
 #include <unistd.h>
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#undef _GNU_SOURCE
+#else
+#include <dlfcn.h>
+#endif
 
 #if BACKWARD_HAS_BFD == 1
 //              NOTE: defining PACKAGE{,_VERSION} is required before including
@@ -225,13 +232,6 @@
 #define PACKAGE_VERSION
 #endif
 #include <bfd.h>
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#include <dlfcn.h>
-#undef _GNU_SOURCE
-#else
-#include <dlfcn.h>
-#endif
 #endif
 
 #if BACKWARD_HAS_DW == 1
@@ -246,13 +246,6 @@
 #include <libdwarf.h>
 #include <libelf.h>
 #include <map>
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#include <dlfcn.h>
-#undef _GNU_SOURCE
-#else
-#include <dlfcn.h>
-#endif
 #endif
 
 #if (BACKWARD_HAS_BACKTRACE == 1) || (BACKWARD_HAS_BACKTRACE_SYMBOL == 1)