Use modern CMake targets for SDL2 (#6132)

src/CMakeLists.txt

@@ -61,9 +61,7 @@ INCLUDE_DIRECTORIES(
 	${FFTW3F_INCLUDE_DIRS}
 )
 
-IF(NOT ("${SDL2_INCLUDE_DIR}" STREQUAL ""))
-	INCLUDE_DIRECTORIES("${SDL2_INCLUDE_DIR}")
-ELSEIF(NOT ("${SDL_INCLUDE_DIR}" STREQUAL ""))
+IF(NOT LMMS_HAVE_SDL2 AND NOT ("${SDL_INCLUDE_DIR}" STREQUAL ""))
 	INCLUDE_DIRECTORIES("${SDL_INCLUDE_DIR}")
 ENDIF()
 
@@ -208,6 +206,11 @@ IF(LMMS_BUILD_WIN32)
 	SET_TARGET_PROPERTIES(lmms PROPERTIES
 		ENABLE_EXPORTS ON
 	)
+	IF(NOT MSVC)
+		SET_PROPERTY(TARGET lmms
+			APPEND_STRING PROPERTY LINK_FLAGS " -mwindows"
+		)
+	ENDIF()
 	IF(LMMS_BUILD_MSYS)
 		# ENABLE_EXPORTS property has no effect in some MSYS2 configurations.
 		# Add the linker flag manually to create liblmms.dll.a import library