build: ensure HAVE_WIN32_SMTC is always defined

It makes sense to only descend into the subdirectory meson build file on
windows, but we should always make sure HAVE_WIN32_SMTC is 0 for other
platforms. Otherwise, it gets unneccesarily awkward to work with.

meson.build

@@ -530,8 +530,11 @@ if not posix and not features['win32-desktop']
                      'osdep/terminal-dummy.c')
 endif
 
+# media controls
 if win32
     subdir('osdep/win32')
+else
+    features += {'win32-smtc': false}
 endif
 
 features += {'glob-posix': cc.has_function('glob', prefix: '#include <glob.h>')}

player/main.c

@@ -399,7 +399,7 @@ int mp_initialize(struct MPContext *mpctx, char **options)
     cocoa_set_mpv_handle(ctx);
 #endif
 
-#if defined(HAVE_WIN32_SMTC) && HAVE_WIN32_SMTC
+#if HAVE_WIN32_SMTC
     if (opts->media_controls == 2 || (mpctx->is_cli && opts->media_controls == 1))
         mp_smtc_init(mp_new_client(mpctx->clients, "SystemMediaTransportControls"));
 #endif