configure.ac: disable the mikmod plugin by default

libmikmod seems to be unmaintained, and has several critical bugs
which make MPD crash.

 http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=461519
 http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=476339

Disable the plugin by default to minimize harm.  Users should
explicitly enable the mikmod decoder (--enable-mod) if they wish to
have it anyway.

configure.ac

@@ -220,7 +220,7 @@ AC_ARG_ENABLE(mod,
 	AS_HELP_STRING([--enable-mod],
 		[enable MOD support (default: disable)]),
 	enable_mod=$enableval,
-	enable_mod=yes)
+	enable_mod=no)
 
 AC_ARG_ENABLE(mpc,
 	AS_HELP_STRING([--disable-mpc],