Silence unused variable warning in Boost

[WardBrian]

Summary

Closes #3017

Tests

None

Side Effects

None

Release notes

Silenced an unused variable warning in Boost.

Checklist

• Copyright holder: Simons Foundation

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[andrjohns]

I've opened a PR upstream with a different fix: boostorg/math#1078

Which is just changing the assert to:

      BOOST_MATH_ASSERT(
         std::numeric_limits<T>::epsilon() < a
         && a < std::numeric_limits<T>::infinity());

I'm happy to either approve and merge this DISABLE_ASSERTS approach now, or wait until upstream comments on that fix and use that instead, just let me know

[WardBrian]

Either seems fine with me

[andrjohns]

Just going to merge this while I sort out some quirks with the upstream PR