The most recent libmad release does not use pkgconfig, and while some
distributors have gone ahead and provided a .pc file, not all have.

This fixes it for the non-pkgconfig case.

Index: flow/gsl/CMakeLists.txt
===================================================================
--- flow/gsl/CMakeLists.txt	(revision 1242076)
+++ flow/gsl/CMakeLists.txt	(working copy)
@@ -63,7 +63,11 @@
   if( MAD_FOUND )
     set( GSL_HAVE_LIBMAD 1 )
   else( MAD_FOUND )
-    message(FATAL_ERROR "\nMAD support are requested, but `libmad` not found" )
+    find_library( MAD_LIBRARIES NAMES mad )
+    find_path( MAD_INCLUDE_DIRS mad.h )
+    if( NOT MAD_LIBRARIES )
+        message(FATAL_ERROR "\nMAD support is requested, but `libmad` not found" )
+    endif( NOT MAD_LIBRARIES )
   endif( MAD_FOUND )
 
 endif( WITH_MAD )