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.

NEWS

@@ -15,6 +15,7 @@ ver 0.14.0 (????/??/??)
   - ffmpeg: new plugin
   - wavpack: new plugin
   - aac: stream support added
+  - mod: disabled by default due to critical bugs in all libmikmod versions
 * commands:
   - "addid" takes optional second argument to specify position
   - "idle" notifies the client when a notable change occurs