Fix #154: 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.

Thanks to this, one can build a .deb package easily with tools such as fpm[1]
with a script like this[2], inside a parallel Mono environment [3].

[1] https://github.com/jordansissel/fpm
[2] https://github.com/knocte/7digital-mono/blob/master/build-fsharp-package.bash
[3] http://www.mono-project.com/Parallel_Mono_Environments