ledger: Use Boost Python support if applicable

Since 772eef91

 Boost no longer has
Python support enabled by default, so depending on whether Ledger has
Python support built in we also use either Boost with Python support or
without.

Tested building with and without Python support and both builds now
succeed.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @the-kenny, @jwiegley