Errata corrige in mixer

This is for Sam:

In makefiles generated for cross-compiling the mixer library
there are errors in variable ${prefix} that is typed {prefix}
and prefix is not automatically set to
/usr/local/cross-tools/i386-mingw32
but remains /usr/local

to compile it correctly I needed to edit all Makefiles

I hope this will help.

Bye,
Enzo