added prefix to header macros

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

src/compress.h

@@ -20,8 +20,8 @@
  *
  */
 
-#ifndef COMPRESS_H
-#define COMPRESS_H
+#ifndef MPD_COMPRESS_H
+#define MPD_COMPRESS_H
 
 /* These are copied from the AudioCompress config.h, mainly because CompressDo
  * needs GAINSHIFT defined.  The rest are here so they can be used as defaults