platform_darwin: runtime-guard URLForResource: for pre-10.6 Leopard

On 10.5 Leopard, RetroArch crashes at startup during default-dir
setup:

    [NSBundle URLForResource:withExtension:subdirectory:]:
        unrecognized selector sent to instance 0x632d410

-[NSBundle URLForResource:withExtension:subdirectory:] is annotated
NS_AVAILABLE(10_6, 4_0) in the 10.8 SDK header - added in Snow
Leopard as part of NSBundle's URL-based API.  10.5 NSBundle only
has the older pathForResource:-style methods returning NSString *.

Same class of issue as setHelpMenu: (aa5c926): compile succeeds
because messages to id dispatch dynamically, but the runtime can't
resolve the selector and throws NSInvalidArgumentException.  Use
the same pre-Obj-C-2.0 idiom:

    if ([[NSBundle mainBundle] respondsToSelector:
            @selector(URLForResource:withExtension:subdirectory:)])
       url = [[NSBundle mainBundle] URLForResource:...];

Both callers in this file (DEFAULT_DIR_AUDIO_FILTER and
DEFAULT_DIR_VIDEO_FILTER setup) already have a fill_pathname_join
fallback for the url==nil case, so the guard just needs to keep
url nil on 10.5.  Cache the SEL in a local since both calls use
the same selector.

Behavioral impact on Leopard: no bundle-shipped filter auto-
discovery; audio/video filter dirs always resolve to
application_data/filters/{audio,video} via the existing fallback.
That's RetroArch's conventional path anyway - the URLForResource:
trick is only an optimization for bundles that ship filters inline,
which is rare on desktop macOS.  Modern macOS behavior unchanged.

Other NSBundle methods in this file (objectForInfoDictionaryKey:
at lines 392, 491) are 10.0-era and need no guard.

Author: LibretroAdmin

frontend/drivers/platform_darwin.m

@@ -444,12 +444,23 @@ static void frontend_darwin_get_env(int *argc, char *argv[],
 #endif
    fill_pathname_join(g_defaults.dirs[DEFAULT_DIR_DATABASE], application_data, "database/rdb", sizeof(g_defaults.dirs[DEFAULT_DIR_DATABASE]));
    fill_pathname_join(g_defaults.dirs[DEFAULT_DIR_CORE_ASSETS], application_data, "downloads", sizeof(g_defaults.dirs[DEFAULT_DIR_CORE_ASSETS]));
-   NSURL *url = [[NSBundle mainBundle] URLForResource:nil withExtension:@"dsp" subdirectory:@"filters/audio"];
+   /* -[NSBundle URLForResource:withExtension:subdirectory:] is 10.6+
+    * (NS_AVAILABLE(10_6, 4_0)).  On 10.5 Leopard the selector doesn't
+    * exist and the runtime throws "unrecognized selector".  Guard
+    * with respondsToSelector: and fall through to the existing
+    * fill_pathname_join fallback on older systems, which simply
+    * won't do bundle-shipped filter auto-discovery. */
+   NSURL *url = nil;
+   SEL url_for_resource_sel = @selector(URLForResource:withExtension:subdirectory:);
+   if ([[NSBundle mainBundle] respondsToSelector:url_for_resource_sel])
+      url = [[NSBundle mainBundle] URLForResource:nil withExtension:@"dsp" subdirectory:@"filters/audio"];
    if (url)
        strlcpy(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER], [[url baseURL] fileSystemRepresentation],  sizeof(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER]));
    else
        fill_pathname_join(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER], application_data, "filters/audio", sizeof(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER]));
-   url = [[NSBundle mainBundle] URLForResource:nil withExtension:@"filt" subdirectory:@"filters/video"];
+   url = nil;
+   if ([[NSBundle mainBundle] respondsToSelector:url_for_resource_sel])
+      url = [[NSBundle mainBundle] URLForResource:nil withExtension:@"filt" subdirectory:@"filters/video"];
    if (url)
        strlcpy(g_defaults.dirs[DEFAULT_DIR_VIDEO_FILTER], [[url baseURL] fileSystemRepresentation],  sizeof(g_defaults.dirs[DEFAULT_DIR_VIDEO_FILTER]));
    else