meson.build: Allow to disable OSS again

If sys/soundcard.h is available, it is currently not possible to
disable OSS with the --disable-oss or --without-default-features
configure switches. Improve the check in meson.build to fix this.

Fixes: 87430d5b13 ("configure, meson: move audio driver detection to Meson")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211102105822.773131-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -911,7 +911,7 @@ if liblzfse.found() and not cc.links('''
 endif
 
 oss = not_found
-if not get_option('oss').auto() or have_system
+if have_system and not get_option('oss').disabled()
   if not cc.has_header('sys/soundcard.h')
     # not found
   elif targetos == 'netbsd'