Make the boost::iostreams dependency optional

[ptosco]

This small PR makes the boost::iostreams dependency optional, such that RDKit can still be built on Windows (MSVC) with CoordGen support without linking to boost::iostreams (e.g., when building CFFI with limited dependencies).
Currently it is possible to build CFFI with CoordGen support with g++ and clang++ as boost::iostreams defines its symbols as weak symbols, but not with MSVC as it does not understand weak symbols.

[ptosco]

@ricrogz @d-b-w Hi both, could one of you take a look at this and merge if you think it makes sense? Thank you very much in advance!

[ricrogz]

LGTM

[d-b-w]

lgtm

[ptosco]

@ricrogz @d-b-w Thank you very much!

[ptosco]

@ricrogz @d-b-w Would you consider cutting a new maeparser 1.3.2 release with this small change? I have just verified that indeed it is now possible to build RDKit MinimalLib without boost::iostreams dependency but with maeparser and coordgen under Windows with this small change, while with 1.3.1 it fails.

[ptosco]

@ricrogz @d-b-w Apologies for stalking you - Would you consider issuing a new maeparser 1.3.2 release incorporating the small change in this PR? That would enable building RDKit MinimalLib without boost::iostreams dependency but with maeparser and coordgen under Windows.
Could you please let me know? If you are not planning a new release just for this change, I will add a patch on the RDKit side.
Many thanks in advance for your reply.