Fix DESTDIR usage in makefiles: $libdir is preferred to $monodir

When building fsharp in a parallel mono environment (i.e.: /opt/mono),
but to a standard prefix such as /usr (for instance, not to install in
this prefix but to create a package via "make install DESTDIR=/tmp/foo"),
the assemblies were ending up in $DESTDIR/opt/mono instead of in
$DESTDIR/usr/lib/mono.