Don't link against both release and debug Boost libs.

On Linux/Mac, release and debug Boost libs are generally
the same, so removing duplicates takes care of
this already. But on Windows, they are usually
different (e.g. boost_system-vc100-mt-1_53.dll
and boost_system-vc100-mt-gd-1_53.dll). Linking
against both is probably not a good idea (in practice
it seems the linker chooses one, seemingly randomly,
so some IMP modules use the release runtime and others
the debug). For now, just link against the release
Boost libraries (on Linux/Mac this won't make any
difference anyway).