fix issue bombela#69

BackwardConfig.cmake

@@ -111,35 +111,45 @@ if (NOT BACKWARD_DEFINITIONS)
 	map_definitions("STACK_DETAILS_" "BACKWARD_HAS_" BACKTRACE_SYMBOL DW BFD)
 endif()
 
-foreach(def ${BACKWARD_DEFINITIONS})
-	message(STATUS "${def}")
-endforeach()
-
 find_path(BACKWARD_INCLUDE_DIR backward.hpp PATHS ${CMAKE_CURRENT_LIST_DIR})
 
+set(BACKWARD_HAS_EXTERNAL_LIBRARIES FALSE)
+set(FIND_PACKAGE_REQUIRED_VARS BACKWARD_INCLUDE_DIR)
+if(DEFINED BACKWARD_LIBRARIES)
+  set(BACKWARD_HAS_EXTERNAL_LIBRARIES TRUE)
+  list(APPEND FIND_PACKAGE_REQUIRED_VARS BACKWARD_LIBRARIES)
+endif()
+
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(Backward
-    REQUIRED_VARS
-        BACKWARD_INCLUDE_DIR
-        BACKWARD_LIBRARIES
+  REQUIRED_VARS ${FIND_PACKAGE_REQUIRED_VARS}
 )
 list(APPEND _BACKWARD_INCLUDE_DIRS ${BACKWARD_INCLUDE_DIR})
 
 macro(add_backward target)
 	target_include_directories(${target} PRIVATE ${_BACKWARD_INCLUDE_DIRS})
 	set_property(TARGET ${target} APPEND PROPERTY COMPILE_DEFINITIONS ${BACKWARD_DEFINITIONS})
-	set_property(TARGET ${target} APPEND PROPERTY LINK_LIBRARIES ${BACKWARD_LIBRARIES})
+	if(BACKWARD_HAS_EXTERNAL_LIBRARIES)
+	  set_property(TARGET ${target} APPEND PROPERTY LINK_LIBRARIES ${BACKWARD_LIBRARIES})
+  endif()
 endmacro()
+
 set(BACKWARD_INCLUDE_DIRS ${_BACKWARD_INCLUDE_DIRS} CACHE INTERNAL "_BACKWARD_INCLUDE_DIRS")
 set(BACKWARD_DEFINITIONS ${BACKWARD_DEFINITIONS} CACHE INTERNAL "BACKWARD_DEFINITIONS")
-set(BACKWARD_LIBRARIES ${BACKWARD_LIBRARIES} CACHE INTERNAL "BACKWARD_LIBRARIES")
+if(DEFINED BACKWARD_LIBRARIES)
+  set(BACKWARD_LIBRARIES ${BACKWARD_LIBRARIES} CACHE INTERNAL "BACKWARD_LIBRARIES")
+endif()
 mark_as_advanced(_BACKWARD_INCLUDE_DIRS BACKWARD_DEFINITIONS BACKWARD_LIBRARIES)
 
 if (NOT TARGET Backward::Backward)
 	add_library(Backward::Backward INTERFACE IMPORTED)
 	set_target_properties(Backward::Backward PROPERTIES
-        INTERFACE_INCLUDE_DIRECTORIES "${_BACKWARD_INCLUDE_DIRS}"
-        INTERFACE_LINK_LIBRARIES "${BACKWARD_LIBRARIES}"
-        INTERFACE_COMPILE_DEFINITIONS "${BACKWARD_DEFINITIONS}"
+    INTERFACE_INCLUDE_DIRECTORIES "${_BACKWARD_INCLUDE_DIRS}"
+    INTERFACE_COMPILE_DEFINITIONS "${BACKWARD_DEFINITIONS}"
+  )
+  if(BACKWARD_HAS_EXTERNAL_LIBRARIES)
+   	set_target_properties(Backward::Backward PROPERTIES
+      INTERFACE_LINK_LIBRARIES "${BACKWARD_LIBRARIES}" 
     )
+  endif()
 endif()