ARROW-4251: [C++][Release] Add option to set ARROW_BOOST_VENDORED environment variable in verify-release-candidate.sh

[wesm]

I'm taking this for a spin on 0.12.1 RC0 on Ubuntu 14.04 (where the system boost does not work -- see ARROW-4868)

[wesm]

This works for me on Ubuntu 14.04, except that I hit ARROW-4250

109: [ RUN      ] TestProjector.TestExtendedMath
109: /tmp/arrow-0.12.1.cZzB4/apache-arrow-0.12.1/cpp/src/gandiva/tests/projector_test.cc:358: Failure
109: Value of: (expected_cbrt)->Equals(outputs.at(0))
109:   Actual: false
109: Expected: true
109: expected array: [
109:   2.51984,
109:   2.15443,
109:   -2.41014,
109:   2.02469
109: ] actual array: [
109:   2.51984,
109:   2.15443,
109:   -2.41014,
109:   2.02469
109: ]

[wesm]

+1