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ARROW-6782: [C++] Do not require Boost for minimal C++ build

With these changes, the minimal build now only requires double-conversion_ep. We still have to disable a number of components manually to obtain the minimal build, so there is more work to do

Closes #5611 from wesm/ARROW-6782 and squashes the following commits:

65b5faec6 <Wes McKinney> Boost also required for Gandiva
6f67a7eac <Wes McKinney> Do not require Boost for minimal C++ build

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>