The build system tries to link to installed system libraries first, which 
fails because the ABI has changed...

Index: mlt-0.8.6/src/swig/ruby/build
===================================================================
--- mlt-0.8.6.orig/src/swig/ruby/build
+++ mlt-0.8.6/src/swig/ruby/build
@@ -5,8 +5,10 @@ if ARGV.shift == 'clean'
   exit 0
 end
 system( "ln -sf ../mlt.i" )
+system( "ln -sf ../../framework/libmlt.so" )
+system( "ln -sf ../../mlt++/libmlt++.so" )
 system( "swig -c++ -ruby -I../../mlt++ -I../.. mlt.i" )
 $CFLAGS += " -I../.. " + ENV['CXXFLAGS']
-$LDFLAGS += " -L../../mlt++ -lmlt++"
+$LIBS += " -lmlt++ -lmlt -lstdc++"
 create_makefile('mlt')
 system( "make" )