Option only changes whether the dep is required and doesn't disable it.
--- a/desmume/src/frontend/posix/meson.build
+++ b/desmume/src/frontend/posix/meson.build
@@ -176,5 +176,5 @@
 endif
 
-if dep_openal.found()
+if dep_openal.found() and get_option('openal')
   dependencies += dep_openal
   libdesmume_src += [