Add handling of the removal of std::unary_function in C++17

[srherbener]

Description

The C++17 standard specifies that std::unary_function is to be removed. Boost has #define directives to handle this, but this requires defining the cxxflag BOOST_NO_CXX98_FUNCTION_BASE on the build (b2) command line. This PR adds in a check for C++17 or newer and adds in the define mentioned above.

This was discovered when trying to build on the Mac using the new [email protected] version. Apparently Apple did not remove std::unary_function until version 15.0.0 was released.

Issue(s) addressed

None

Dependencies

List the other PRs that this PR is dependent on:
None

Impact

Expected impact on downstream repositories:
None

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (N/A)
• I have run the unit tests before creating the PR (tested manually on my Mac)

[climbfuji]

Also here it would be good to cherry-pick the updates for boost from spack develop and then applying the changes for all versions.

[srherbener]

After fixing the syntax error, I re-tested on my Mac and boost built successfully.

[srherbener]

After fixing the coding style error, I re-tested on my Mac and boost built successfully.

[climbfuji]

CI tests passed for the associated spack-stack PR.