CMake: Guard target creation in exported config (#464)

User projects may call 'find_package(c-ares)' multiple times (e.g.
via dependencies), but targets must be created only once.
Shared and static target must be treated independently.

Fix By: Kai Pastor (@dg0yt)

c-ares-config.cmake.in

@@ -8,14 +8,17 @@ include("${CMAKE_CURRENT_LIST_DIR}/c-ares-targets.cmake")
 set(c-ares_LIBRARY c-ares::cares)
 
 if(@CARES_SHARED@)
-	add_library(c-ares::cares_shared INTERFACE IMPORTED)
-	set_target_properties(c-ares::cares_shared PROPERTIES INTERFACE_LINK_LIBRARIES "c-ares::cares")
+	if(NOT TARGET c-ares::cares_shared)
+		add_library(c-ares::cares_shared INTERFACE IMPORTED)
+		set_target_properties(c-ares::cares_shared PROPERTIES INTERFACE_LINK_LIBRARIES "c-ares::cares")
+	endif()
 	set(c-ares_SHARED_LIBRARY c-ares::cares_shared)
-elseif(@CARES_STATIC@)
-	add_library(c-ares::cares_static INTERFACE IMPORTED)
-	set_target_properties(c-ares::cares_static PROPERTIES INTERFACE_LINK_LIBRARIES "c-ares::cares")
 endif()
 
 if(@CARES_STATIC@)
+	if(NOT TARGET c-ares::cares_static)
+		add_library(c-ares::cares_static INTERFACE IMPORTED)
+		set_target_properties(c-ares::cares_static PROPERTIES INTERFACE_LINK_LIBRARIES "c-ares::cares")
+	endif()
 	set(c-ares_STATIC_LIBRARY c-ares::cares_static)
 endif()