Find dependencies for statically-linked libdw, try to use pkg-config

BackwardConfig.cmake was not explicitly pulling in the
dependencies (libelf, libz, etc.) of libdw, which made it fail to link
when libdw was built as a static library (and therefore didn't
explicitly encode those dependencies).

This changes it to first try using pkg-config to find libdw and its
dependencies, and then fall back to looking up the individual
libraries it might depend on.

BackwardConfig.cmake

@@ -53,6 +53,10 @@ endif()
 # CONFIGS
 ###############################################################################
 include(FindPackageHandleStandardArgs)
+if(NOT PKG_CONFIG_FOUND)
+	include(CMakeFindDependencyMacro)
+	find_dependency(PkgConfig)
+endif()
 
 if (STACK_WALKING_LIBUNWIND)
 	# libunwind works on the macOS without having to add special include
@@ -87,12 +91,30 @@ if (${STACK_DETAILS_AUTO_DETECT})
 		set(_name_mismatched_arg NAME_MISMATCHED)
 	endif()
 	# find libdw
-	find_path(LIBDW_INCLUDE_DIR NAMES "elfutils/libdw.h" "elfutils/libdwfl.h")
-	find_library(LIBDW_LIBRARY dw)
-	set(LIBDW_INCLUDE_DIRS ${LIBDW_INCLUDE_DIR} )
-	set(LIBDW_LIBRARIES ${LIBDW_LIBRARY} )
-	find_package_handle_standard_args(libdw ${_name_mismatched_arg}
-		REQUIRED_VARS LIBDW_LIBRARY LIBDW_INCLUDE_DIR)
+	# start by trying pkg-config
+	pkg_search_module(LIBDW libdw)
+	if(NOT LIBDW_FOUND)
+		find_path(LIBDW_INCLUDE_DIR NAMES "elfutils/libdw.h" "elfutils/libdwfl.h")
+		find_library(LIBDW_LIBRARY dw)
+		# if it's statically linked, look for all the possible dependencies
+		find_library(LIBELF_LIBRARY elf)
+		find_library(LIBZ_LIBRARY z)
+		find_library(LIBBZ2_LIBRARY bz2)
+		find_library(LIBLZMA_LIBRARY lzma)
+		find_library(LIBZSTD_LIBRARY zstd)
+		set(LIBDW_INCLUDE_DIRS ${LIBDW_INCLUDE_DIR} )
+		set(LIBDW_LIBRARIES ${LIBDW_LIBRARY}
+			$<$<BOOL:${LIBELF_LIBRARY}>:${LIBELF_LIBRARY}>
+			$<$<BOOL:${LIBZ_LIBRARY}>:${LIBZ_LIBRARY}>
+			$<$<BOOL:${LIBBZ2_LIBRARY}>:${LIBBZ2_LIBRARY}>
+			$<$<BOOL:${LIBLZMA_LIBRARY}>:${LIBLZMA_LIBRARY}>
+			$<$<BOOL:${LIBZSTD_LIBRARY}>:${LIBZSTD_LIBRARY}>)
+		find_package_handle_standard_args(libdw ${_name_mismatched_arg}
+			REQUIRED_VARS LIBDW_LIBRARY LIBDW_INCLUDE_DIR)
+	endif()
+	# libdw doesn't declare its pthread dependency
+	find_library(LIBPTHREAD_LIBRARY pthread)
+	set(LIBDW_LIBRARIES ${LIBDW_LIBRARIES} ${LIBPTHREAD_LIBRARY})
 	mark_as_advanced(LIBDW_INCLUDE_DIR LIBDW_LIBRARY)
 
 	# find libbfd