Silence complaints from FindSDL2 cmake module

Added aminosbh/sdl2-cmake-modules#6

cmake/FindSDL2.cmake

@@ -42,13 +42,15 @@ This module defines the following 'IMPORTED' targets:
 
 ::
 
-  SDL2::Core
+  SDL2::Core (for compatibility with older versions)
+  SDL2::SDL2 (compatibility with CONFIG mode)
     The SDL2 library, if found.
-    Libraries should link to SDL2::Core
+    Libraries should link to SDL2::SDL2
 
-  SDL2::Main
+  SDL2::Main (for compatibility with older versions)
+  SDL2::SDL2main (compatibility with CONFIG mode)
     The SDL2main library, if found.
-    Applications should link to SDL2::Main instead of SDL2::Core
+    Applications should link to SDL2::SDL2main instead of SDL2::SDL2
 
 
 
@@ -310,16 +312,14 @@ endif()
 
 include(FindPackageHandleStandardArgs)
 
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(SDL2
-                                  REQUIRED_VARS SDL2_LIBRARY SDL2_INCLUDE_DIR
-                                  VERSION_VAR SDL2_VERSION_STRING)
-
+set(SDL2_REQUIRED_VARS SDL2_LIBRARY SDL2_INCLUDE_DIR)
 if(SDL2MAIN_LIBRARY)
-  FIND_PACKAGE_HANDLE_STANDARD_ARGS(SDL2main
-                                    REQUIRED_VARS SDL2MAIN_LIBRARY SDL2_INCLUDE_DIR
-                                    VERSION_VAR SDL2_VERSION_STRING)
+  list(APPEND SDL2_REQUIRED_VARS SDL2MAIN_LIBRARY SDL2_INCLUDE_DIR)
 endif()
 
+find_package_handle_standard_args(SDL2
+                                  REQUIRED_VARS ${SDL2_REQUIRED_VARS}
+                                  VERSION_VAR SDL2_VERSION_STRING)
 
 mark_as_advanced(SDL2_PATH
                  SDL2_NO_DEFAULT_PATH
@@ -384,5 +384,9 @@ if(SDL2_FOUND)
                    INTERFACE_LINK_LIBRARIES SDL2::MainInternal)
     endif()
 
+    # compatibility targets
+    add_library(SDL2::SDL2 ALIAS SDL2::Core)
+    add_library(SDL2::SDL2main ALIAS SDL2::Main)
+
   endif()
 endif()