CMake: Only find and link to SDL when needed

src/unix/CMakeLists.txt

@@ -32,6 +32,16 @@ set(THREADS_PREFER_PTHREAD_FLAG TRUE)
 find_package(Threads REQUIRED)
 target_link_libraries(86Box Threads::Threads)
 
+find_package(SDL2 REQUIRED)
+include_directories(${SDL2_INCLUDE_DIRS})
+if(STATIC_BUILD AND TARGET SDL2::SDL2-static)
+    target_link_libraries(86Box SDL2::SDL2-static)
+elseif(TARGET SDL2::SDL2)
+    target_link_libraries(86Box SDL2::SDL2)
+else()
+    target_link_libraries(86Box ${SDL2_LIBRARIES})
+endif()
+
 add_library(ui OBJECT
     unix_sdl.c
     unix_cdrom.c