package/stella: libpng is optional, not mandatory

libpng is optional, not mandatory since version 6.1 and
https://github.com/stella-emu/stella/commit/eb13d515fb6385df6c653c5d295d97979b140031

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/stella/Config.in

@@ -6,7 +6,6 @@ config BR2_PACKAGE_STELLA
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_6 # C++14
 	depends on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735 # exception_ptr
 	select BR2_PACKAGE_SDL2
-	select BR2_PACKAGE_LIBPNG
 	select BR2_PACKAGE_ZLIB
 	help
 	  Stella is a multi-platform Atari 2600 VCS emulator.

package/stella/stella.mk

@@ -10,13 +10,20 @@ STELLA_SITE = https://github.com/stella-emu/stella/releases/download/$(STELLA_VE
 STELLA_LICENSE = GPL-2.0+
 STELLA_LICENSE_FILES = Copyright.txt License.txt
 
-STELLA_DEPENDENCIES = sdl2 libpng zlib
+STELLA_DEPENDENCIES = sdl2 zlib
 
 STELLA_CONF_OPTS = \
 	--host=$(GNU_TARGET_NAME) \
 	--prefix=/usr \
 	--with-sdl-prefix=$(STAGING_DIR)/usr
 
+ifeq ($(BR2_PACKAGE_LIBPNG),y)
+STELLA_CONF_OPTS += --enable-png
+STELLA_DEPENDENCIES += libpng
+else
+STELLA_CONF_OPTS += --disable-png
+endif
+
 # The configure script is not autoconf based, so we use the
 # generic-package infrastructure
 define STELLA_CONFIGURE_CMDS