Use LIBDIR instead of libdir for textual substitution

@libdir@ can contain further macros.

configure.ac

@@ -440,6 +440,7 @@ AC_SUBST(TESTDIR)
 AC_SUBST(EXAMPLESDIR)
 AC_SUBST(EXTRAS)
 AC_SUBST(BINDIR)
+AC_SUBST(LIBDIR)
 AC_SUBST(PKGDATADIR)
 AC_SUBST(PKGEXTRABINDIR)
 AC_SUBST(LAUNCHSTYLE)

music-config/predict_rank.py.in

@@ -17,7 +17,7 @@ def c_argc_argv (argv):
 
     return (c_argc, c_argv)
 
-libmusic = ctypes.CDLL ("@libdir@/libmusic-c.so")
+libmusic = ctypes.CDLL ("@LIBDIR@/libmusic-c.so")
 
 def predictRank ():
     return libmusic.MUSIC_predictRank (*c_argc_argv (sys.argv))