cmake: fix Windows linking

src/sdl/CMakeLists.txt

@@ -113,10 +113,15 @@ if(${SDL2_FOUND})
 			${PNG_LIBRARIES}
 			${ZLIB_LIBRARIES}
 			${OPENGL_LIBRARIES}
-			m #need math lol
-			rt # this too
 		)
 
+		if(${CMAKE_SYSTEM} MATCHES Linux)
+			target_link_libraries(${SRB2_SDL2_EXE_NAME} PRIVATE
+				m
+				rt
+			)
+		endif()
+
 	endif()
 
 	set_target_properties(${SRB2_SDL2_EXE_NAME} PROPERTIES VERSION ${SRB2_VERSION})