Make flac dependency optional by providing a --without-flac option, as
requested in bug #114228.

Index: libsndfile-1.0.12/configure.ac
===================================================================
--- libsndfile-1.0.12.orig/configure.ac
+++ libsndfile-1.0.12/configure.ac
@@ -322,15 +322,19 @@ if test x$ac_cv_header_alsa_asoundlib_h 
 #====================================================================================
 # Check for FLAC
 
-FLAC_LIBS=""
-AC_CHECK_HEADERS(FLAC/all.h)
-if test x$ac_cv_header_FLAC_all_h = xyes ; then
+AC_ARG_WITH([flac], AC_HELP_STRING([--without-flac], [Disable FLAC support (default: check)]))
+
+if test "x$with_flac" != "xno"; then
+  FLAC_LIBS=""
+  AC_CHECK_HEADERS(FLAC/all.h)
+  if test x$ac_cv_header_FLAC_all_h = xyes ; then
         AC_CHECK_LIB(FLAC, FLAC__seekable_stream_encoder_set_tell_callback, HAVE_FLAC_1_1_1="yes")
         if test "x$HAVE_FLAC_1_1_1" = xyes ; then
            AC_DEFINE(HAVE_FLAC_1_1_1, [1], [Define to 1 if you have libflac 1.1.1])
            fi
 	FLAC_LIBS="-lFLAC"
 	fi
+fi
 
 #====================================================================================
 # Test for sanity when cross-compiling.