SDL_mixer: cmake: (Re-)fix building with SDL < 2.0.12

From 0cb7dc5c3a4a7ffd24823198a5c26adffe04bc56 Mon Sep 17 00:00:00 2001
From: Charlie Birks <[EMAIL REDACTED]>
Date: Tue, 14 Dec 2021 10:31:22 +0000
Subject: [PATCH] cmake: (Re-)fix building with SDL < 2.0.12

Which is what the action build uses.
---
 CMakeLists.txt | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ea894966..d699e35b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -21,6 +21,15 @@ option(BUILD_SHARED_LIBS "Enable shared library" ON)
 
 if (NOT ANDROID AND NOT (TARGET SDL2 OR TARGET SDL2-static))
 	find_package(SDL2 REQUIRED)
+
+	if(NOT TARGET SDL2::SDL2)
+	    # SDL < 2.0.12
+	    add_library(SDL2::SDL2 INTERFACE IMPORTED)
+        set_target_properties(SDL2::SDL2 PROPERTIES
+            INTERFACE_INCLUDE_DIRECTORIES ${SDL2_INCLUDE_DIRS} ${SDL2_INCLUDE_DIR}
+            INTERFACE_LINK_LIBRARIES ${SDL2_LIBRARIES} ${SDL2_LIBRARY}
+        )
+	endif()
 endif()
 
 include_directories(include src src/codecs)