v.pref: only show the `disabling parallel cgen` message when -no-parallel was not given and -prealloc was

vlib/v/pref/default.v

@@ -137,7 +137,9 @@ pub fn (mut p Preferences) fill_with_defaults() {
 		p.bare_builtin_dir = os.join_path(p.vroot, 'vlib', 'builtin', 'linux_bare')
 	}
 	$if prealloc {
+		if !p.no_parallel {
 			eprintln('disabling parallel cgen, since V was built with -prealloc')
+		}
 		p.no_parallel = true
 	}
 }