-Added dsp plugin compilation (/dev/dsp is not a regular file :))

configure

@@ -3464,7 +3464,7 @@ if test "${enable_dsp+set}" = set; then
 fi
 
 if test x$enable_dsp != xno; then
-  if test -f /dev/dsp; then
+  if test -c /dev/dsp; then
     PLUGINS=${PLUGINS}"dsp "
   fi
 fi

configure.in

@@ -158,7 +158,7 @@ if test x$enable_null != xno; then PLUGINS=${PLUGINS}"null "; fi
 AC_ARG_ENABLE(dsp,
   [  --disable-dsp           Linux /dev/dsp support (default enabled)])
 if test x$enable_dsp != xno; then
-  if test -f /dev/dsp; then
+  if test -c /dev/dsp; then
     PLUGINS=${PLUGINS}"dsp "
   fi
 fi